STEM Electronics

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Topic Introduction

The field of electronics has changed our lives like no other technical achievement of the human race. This is the case although the field governing electronic components and their interactions is less than 150 years old. Electronics accompany us throughout every moment of our days.

Let's look at a typical everyday schedule:

We wake up in the morning because our electric alarm clock or smartphone wakes us up. We turn on the light. No LED lamp would work without electronics. The microwave heats up our hot chocolate, while the toaster warms our toast - both controlled by electronics. Time to brush our teeth – with an electronically controlled toothbrush, of course. We leave the house. The electronics inside the street lights have already turned off; it’s bright outside. The bus driver pushes a button to release the doors – electronically – and we get on. The trip to school begins. The combustion engine inside the bus today is controlled exclusively by electronics. We could go on and on ...

Most of these things feel like a matter of course to us. Often, we don’t even realise electronics are at play. In addition, today's electronic circuits can be extremely tiny. We don't even see them, unless we look closely. However, in many areas electronics have been able to replace complex and expensive mechanical systems. They use much less energy, and are more robust than other solutions.

Let's look at a common LED lamp used to provide lighting. The LED itself is an electronic component.  A high-efficiency electronic transformer supplies the LED with current from the power grid. Cheap electronics turn 230 V into 3 V. Under these optimal conditions, the LED requires only around 1/10 the energy to light a room just as brightly as an incandescent bulb. And it does so with a service life 10 to 100 longer than the incandescent.  Even the price for LEDs is slowly dropping down to that of traditional light bulbs.

Electric mobility is another example. The drive units inside electric vehicles can achieve an efficiency of up to 80%. A unit with a combustion engine doesn’t even achieve 25%. In addition, electric drives use much fewer expensive mechanical components. For example, they no longer need a transmission and clutch. Power electronics for speed regulation are much cheaper.



Electronics deals with the areas of electrical engineering in which active components are replaced. In active components, current is conducted by a semiconductor or a vacuum (tube technology). The components are able to control the flow of current in the circuits in which they are used. Today, semiconductors are the most important component. Tubes are used in the radio field (lower cost with higher performance) and audio technology (better sound properties). 


When Italian physicist Alessandro Graf von Volta invented the battery in 1800, the first usable power source was created. A reliable supply of electricity allowed research on electricity to truly take off.

1873, three quarters of a century later, Willoughby Smith was able to prove the photoelectric properties of selenium. Just one year later, Ferdinand Braun discovered the rectifying properties of semiconductors. These two discoveries are considered the birth of the field of electronics.

For the first half of the last century, vacuum tubes were the only active component in electronics. In 1904, John Ambrose Fleming introduced a vacuum diode. By adding control grids to the flow of electrons in the next few years, better and better amplifier tubes became available. One unique feature of this technology was certainly the cathode ray tube (Ferdinand Braun, 1897). It was produced for televisions up to the 2010s. Another unique type of tube is the magnetron used in microwave ovens, still an essential feature in every household today.


Julius Edgar Lilienfeld patented an electronic component comparable to today's field effect transistors in 1925. At the time, it had no practical applications. The technical requirements needed to manufacture it were simply not yet in place.

John Bardeen, Walter Brattain and William Shockley from Bell Laboratories introduced the first transistor in 1947, winning the Nobel Prize in physics for it 9 years later.
In the years that followed, semiconductor technology was revolutionised by the use of transistors. Over the 1950s and 1960s, the germanium initially used in these was replaced by silicon. Silicon is easier to obtain, easier to handle, and less expensive. Transistors came to replace tubes more and more often.

In 1958, Jack Kilby developed the first integrated circuit (IC). This was a flip flop made of 2 transistors. Today, over 50 million transistors can be combined on a chip. They are switched with speeds ranging into the gigahertz range.

After 1960, MOSFETs (metal oxide semiconductor field effect transistors) heralded modern power electronics. IGBTs (insulated gate bipolar transistors) were another development. They combine the properties of field effect and bipolar transistors. They can switch voltages over 6000 V, currents well over 3000 ampere, and power levels of up to 100 megawatts using only low control voltages.

Electronics have a long tradition at fischertechnik as well. In 1969, fischerwerke launched its first electronic building set on the market. The goal was to provide a practical way to get familiar with the field of electronics. Many other experimental and model kits followed, even including computing experiments.



Current and voltage

Task 1

Every electronic circuit requires a voltage supply. This voltage is provided by a power source. The strength of the power source is expressed by the voltage at its poles. Voltage is a physical parameter designated by the letter U. The quantity of voltage is indicated in the unit volts (V). The voltage for the battery used here in the battery holder is 9 volt.

Chemical processes inside the battery separate positive and negative charge carriers. The negative charge carriers are the electrons. The positive charge carriers are points from which the electrons have been driven.

The more electrons are separated from their positions, the higher the voltage. Voltage is the force that the electrons exercise between the poles. Voltage is measurable between the two poles of a power source.

If a closed circuit is connected between the two poles of the battery, the electrons can balance out via the power circuit. Current flows through the electric circuit. The voltage drives the current through the electric circuit. The higher the voltage, the greater the current. 

Current is designated by the letter I. Its quantity is indicated in amperes (A). Generally, current flows in electronic circuits are very small. They will only be measured in milliamperes (1 mA = 0.001 A). Even the XS motor used by fischertechnik “draws” only around 100 mA from the voltage source under normal load.

The next section will address the relationship between voltage and current in more detail.

Another important physical parameter is power. An electrical circuit is always the consequence of obstacles to the current. Work must be done on each component (including cables, plugs, switches, ...) to overcome the obstacle. Often, the work being done can be observed directly. In our circuit, for instance, this is the illumination of the LED.However, a turning motor or the heating of components is also work that is being performed. The work performed per time unit, then, is the power.

Power is simple to calculate:
P = U x I Power = Voltage x Current

Electronic components are sensitive to temperature changes. If the component becomes hot, this can change its properties or even destroy it. Therefore, data sheets always include information on maximum power loss.



Task 4

A resistor is a passive component used in electrical engineering and electronics. It causes a certain current to flow at a specified voltage. The electrical resistance R is the physical measurement of a resistor. It is indicated in ohms Ω. Voltage and current are proportionally dependent on an electrical resistor; this is expressed by Ohm's law.

R = U / I Resistance = Voltage / Current
U = R * I Voltage = Resistance x Current
I = U / R Current = Voltage / Resistance

Current and voltage in a resistor are proportionally dependent on one another. This dependency can be represented by a characteristic curve. Figure 7 shows the characteristic curve for a330 Ω resistor. At 9 volt, a current of 0.027 A to 27 mA will flow. The characteristic curve of a resistor is a straight line. It is linear.


The electrical resistance of a component is always dependent on the ambient conditions (such as the temperature). In many electronic circuits, the influences are so low as to be negligible. There are components where influences can be utilised in a targeted manner. Due to the physical properties of the materials used, for instance, they may be very sensitive to light (LDR = light-dependent resistor) or temperature (NTC = negative temperature coefficient thermistor or PTC = positive temperature coefficient thermistor).

The resistors used in this box have the SMD design common today (surface mounted device, Figure) and are made of a ceramic substrate to which a metal or carbon film is applied. Resistors can also be manufactured from wound wires or semiconductor materials. The latter are used in integrated circuits.


Series switching of resistors

Task 5

If resistors are switched in a series, their electric resistances are added together.

R = R1 + R2 + R3 + ...

total resistance = Resistance 1 + Resistance 2 + Resistance 3 + …

If a power source is connected, a voltage is dropped on each resistor proportional to its electrical resistance. This is known as a voltage divider. Voltage dividers are used universally in electronic circuits. Voltage dividers make it possible to operate electronic components in their optimal working ranges. This is called setting the operating point.

Figure 8 shows a voltage divider made of two resistors. The power supply U applied is divided by the two resistors according to their sizes, into U1 and U2. This can be expressed in a formula:


If we look at all voltages in the circuit, we can also see the correlation between the voltage supply U and the partial voltages on the resistors. Since the voltage U is the same as the total voltage U1 and U2, the following formula applies:

U = U1 + U2   

This indicates the relationship to the supply voltage


If the resistances are known (such as R1 = 47 kΩ, R2 = 3,3 kΩ), we can calculate U2, for instance:

U2 = (U x R2)/(R1+ R2) = (9V x 3.3 kΩ)/(47 kΩ + 3.3 kΩ) = 0.59 V

We can also understand the circuit in Figure 8 as a network of voltages. The correlation is expressed by Kirchhoff’s second law (mesh law):
The total of all voltages in a network is zero.

The rule can be shown with the help of a directional arrow indicating the mesh loop. If we follow the entire circuit or network anti-clockwise, we see that the directional arrow for the voltage supply U runs against the direction of rotation. This means that the voltage U must have a negative sign. Therefore, the rule is confirmed:

U1 + U2 – U = 0.

Parallel switching of resistors

Task 6

If resistors are switched in parallel, their electrical conductivity values are added (symbol: G). The conductivity value expresses how well a material conducts electrical current. The conductivity value is the reverse of the electrical resistance. With this knowledge, you can calculate parallel switching of resistors.

1/R=  1/R1+1/R2+1/R3+⋯

The total resistance R is always less than the smallest resistance in the parallel circuit.

When a power source is connected, the same voltage will be applied to all resistors in the parallel circuit. Current flows through each resistor which is inversely proportional to the electrical resistance (proportional to the conductivity value).

Figure 9 Shows a parallel circuit made of two resistors. The total flow of current I is divided by the two resistors according to their sizes, into I1 and I2. This can be expressed in a formula:


If we look at all currents in the circuit, we can also see the correlation between the total current I and the partial currents on the resistors. Since the total current I equals the total of currents I1 ad I2, this results in the following formula:

I = I1 + I2

If the supply voltage and resistances are known (such as U = 9 V, R1 = 47 kΩ, R2 = 3,3 kΩ) then we can calculate the currents using Ohm's law:

I1 = U/R1 = 9V/(47 kΩ) = 0.19 mA I2 = U/R1 = 9V/(3.3 kΩ) = 2.73 mA
I = I1 + I2 = 0.19mA + 2.73 mA = 2,92 mA

We can also understand the circuit in Figure 9 as a network of currents. The correlation is expressed by Kirchhoff’s first law (junction law):
At a network junction, the total of all currents flowing into the junction will equal the total of all currents flowing out.

Looking at the junctions shown on the wiring diagram, I flows into the junctions, while I1 and I2 flow out.

I – I1 – I2 = 0.


A capacitor can store electrical energy. In principle, a capacitor consists of two conductive plates separated by an insulator. The insulator is called the dielectric. The plates and dielectric can be made of many different materials. One example is a film capacitor. Two plastic films and two metal films are stacked on top of one another and rolled up or folded. Each metal foil has a connection that later protrudes from the surrounding housing.

Another important design is the electrolytic capacitor. The dielectric is formed by an insulating oxide layer. Electrolytic capacitors are polarized components. Incorrect polarity will destroy them.

If the capacitor is connected to a power supply, charge carriers flow onto the plates. The capacitor is charged. If the capacitor is disconnected from the power supply, it retains its charge. The charge of commonly available capacitors, however, is so low that they cannot be used to operate electrical devices. An LED would only light up briefly. Very low currents flow in electronic circuits. In these, the charge of a capacitor is sufficient perform many tasks. In the capacitor experiment, the capacitor is discharged slowly via high resistances to show this effect.



Modern electronics are based on semiconductor components.  These components are based on high-purity semiconductor crystals. The crystals are contaminated with suitable materials in a targeted manner. This is called doping. Further doping (n-type doping) is conducted at required areas of the crystals with other materials in order to obtain the desired properties from the component.

There are two types of doping. n-type doping implants atoms into the crystal that have one too many electrons (such as phosphorous, P). These excess electrons can move freely. p-type doping implants atoms with one electron too few (such as boron, B). The points at which electrons are lacking are called holes or electron deficiencies. These points cause the crystal to be conductive. Electrons can move from hole to hole. The boundaries between p-type and n-type doping areas are called p-n junctions.

The conductivity of the crystal increases significantly due to the doping. The electrons don't have anything better to do than plug the holes. They diffuse into the p-doped zone and recombine with the holes. The holes disappear, and seem to diffuse into the n-doped zone. 

This continues to a certain depth (diffusion length) of the material. The process quickly comes to a stop because the movement of the charge carriers creates an electrical field that counteracts the process. A balance is formed. This zone without free charge carriers around the p-n junction is called the barrier layer or depletion region. Because of a lack of charge carriers, current can no longer flow. The electrical field produced results in diffusion voltage. For silicone, this is around 0.7 volts. 

The diffusion voltage must be exceeded by applying an external voltage so that a current can flow through the semiconductor. The p-zone must be more positive than the n-zone. This means that the technical current direction in a p-n junction always runs from p to n, corresponding to the direction of movement for the holes. The electrons move from n to p, therefore, when the current is flowing.



Tasks 7 and 8

One of the simplest semiconductor elements is the diode. It consists of just one p-n junction. Generally, p-doping is implanted at a point on an n-doped semiconductor crystal. The two zones are guided out of the housing via connections. The connection on the p zone is called the anode, and that on the n zone is called the cathode. 

There are two options for adding a silicone diode to a circuit:

The anode is more positive than the cathode, or vice versa. If the anode voltage is more positive by approx. 0.6 – 0.7 Volt, then the diode is operated in the forward direction. The diffusion voltage is dissipated, electrons and holes reverse their recombination, and act as free charge carriers. Current flows.

In the reverse case, the electrical field of the barrier layer and applied voltage work in the same direction. The electrical field is amplified further by the external voltage, the diode is blocked, and current cannot flow. The diode is operated in the barrier direction.

If the voltage is increased too far in the barrier direction, the physical effects inside the semiconductor cause an unwanted breakdown. A high current snowballs through and destroys the diode. The breakdown voltages for the diodes used in the experiments is approx. 700 volts.

LEDs (light emitting diodes) are a unique type of diodes. When they are operated in the forward direction, they convert electrical energy into light. The forward voltage is 1.8 to 3.7 volts, depending on the material and light colour. The reverse voltage is generally around 5 volts.


The transistor

Tasks 9 to 11

The “bipolar transistor” (BJT) or simple transistor is a semiconductor component with two barrier layers. Therefore, there must be two p-n junctions. The doping sequence determines the electrical properties of the transistor. Figure 19 shows the two possible combinations.

If voltage is applied to the transistor between the base and emitter, then the base current flows after the diffusion voltage is exceeded IB. The section acts like a diode. A large number of charge carriers enter the base layer with the base current. Since this is very narrow, more and more charge carriers have to move into the barrier layer between the base and collector. The charge carriers remove the barrier layer and make it conductive. A collector current of IC can flow. The strength of the current depends on the charge carriers present in the base.

A current flows through the collector and emitter that is dependent on the base current.  

The process will stop without a supportive base current. This does not happen abruptly, but rather requires a short amount of time. The charge carriers must flow out of the base. This is why a transistor cannot switch infinitely quickly, but instead has a limit frequency. This places limits on fast applications, such as in computers. Another consequence is that a transistor switches on much more quickly than it switches off.

The ratio between the collector current IC and base current IB is the current amplification factor B. This parameter is stipulated by the manufacturer. Depending on the transistor type, current amplification is between 5 and 1000. The current amplification for the transistors used in the experiments is around 200. 

"Amplification factor B = "  ("Collector current"  "I" _"C" )/("Base current"  "I" _"B"  )


The transistor is a current amplifier. If the base current is increased, the collector current increases. At a certain point, the current is limited by the load resistance. The collector current cannot increase further, even if the base current increases. The transistor is at maximum power. It is operated in saturation mode. This operating mode is used if the transistor will be used as a switch.

If the transistor is used as an amplifier, it must be operated in a range where the collector current is proportionally dependent on the base current.
Different voltages and currents are important in a transistor circuit. They are shown in the Figure.

UB - supply voltage
UBE -  base-emitter voltage
IB - base current
UCE - collector-emitter voltage
IC - collector current
RL - load resistance (for instance a lamp)
RB - base series resistor
B - Current amplification factor

UCB and IE are negligible. They can be calculated from the other values at any time.
It must be ensured that the transistor is operated within the manufacturer’s specifications. The following parameters cannot be exceeded:

UCB0 - Maximum collector-base voltage
UCE0 - Maximum reverse voltage
UEB0 - Maximum base-emitter voltage
IC - Maximum collector current
Ptot Maximum power dissipation (UCE x IC, base current negligible)


Some parameters are shown by characteristic curves, in order to express the dependencies of different values on one another. The most important characteristic curve that describes a transistor is the characteristic output curve (Figure 21). It can be used to read the correlation between the collector-emitter voltage and collector current for different base currents. After reading the collector current, the amplification factor can also be calculated from the line of a base current. The yellow line indicates the range that may not be exceeded. The product of the collector-emitter voltage and collector current indicates the power converted into heat. Beyond the line, this would be too high and would destroy the transistor.

Today, many manufacturers do not indicate all characteristic curves. It is then necessary to use the parameters indicated in the data sheet to calculate circuits. The information in the data sheet can be used to calculate transistor circuits. Only Ohm’s law and Kirchhoff’s laws are needed to do so.

If the resistance of a load (device) is known, the current can be determined by it and thereby by the collector:

UL = UB – UCE, IL = IC = UL / RL

The base current can be calculated by simply dividing the collector current by current amplification factor B:

IB = IC / B

The required base current, in turn, determines the base resistance. Voltage URB drops here. The voltage corresponds to the supply voltage minus the diffusion voltage for the base-emitter section:

URB = UB – 0.7V

If this voltage is known, the base resistance can be calculated:



The Darlington circuit

Task 12

The Darlington circuit is a unique way to connect two transistors (see Figure 22). Base and collector current flowing over the emitter from the transistor are used directly as base current for the second transistor. In this circuit, the current amplification factors of the two transistors multiply one another.

Btotal = BT1 x BT2

Darlington circuits have very high current amplification factors. Factors of over 500,000 can be reached.
Today, Darlington transistors are used whenever Darlington circuits are required. They consist of a Darlington circuit in a transistor housing.



Metal oxide semiconductor field effect transistor
Tasks 13 to 15

MOSFETs have replaced transistors in many areas. This component is used in digital and power electronics, in particular. MOSFETs can be produced extremely efficiently in very high quantities, and can be combined to create complex circuits on a piece of silicone. There are microprocessors that consist of more than 50 billion individual MOSFETs.

In power electronics, MOSFETs can switch very high levels of current at a very low voltage. It is no longer necessary to use mechanical circuit breakers. This is possible thanks to the very good electrical properties of modern MOSFETs.

In principle, a MOSFET is a semiconductor component that acts like a voltage-controlled resistor. In contrast to bipolar transistors, no control voltage flows. The MOSFET is controlled without power.

A MOSFET consists of a semiconductor crystal that can have a weak n or p-type doping. There are specialised types based on an undoped crystal.

An n-channel MOSFET like the one used for experiments consists of a p-doped silicone (substrate) in which two n-zones have been implanted (Figure). The two zones are connected to the drain and source connection. An insulating layer is applied to the area between the zones. The gate electrode is located on the insulating layer. The bulk connection is on the substrate. This is internally connected to the source.

If a positive voltage is applied to the gate (Figure), the electrical field that is produced sucks in electrons. The gate acts like a capacitor towards the substrate. The collected electrons form a channel below the gate electrode that acts like n-doped silicon. The channel connects the two n-zones and eliminates the barrier layers. Current can flow. In modern MOSFETs, the channel has a very low resistance (< 10 mOhm). This allows power dissipation to be kept very low, even with large currents.

Current can flow in both directions between the drain and source. Manufacturers optimise MOSFETs, however, for the direction of flow from Drain - Source.
Switching multiple MOSFET cells in parallel on a substrate can be used to create components that can switch very high currents (> 3000 amperes). 


The MOSFET as a switch

Tasks 14 to 15

MOSFETs for power electronics should have the lowest possible resistance RDS(on) for the channel between the drain and source. Today, they achieve values into the milliohm level.

The charge current ID flows through the load resistor and channel between the drain and source The channel is switched in series with the load resistance RL. Figure 26 shows the relationship in our circuit and as an equivalent circuit. The goal is to allow RDS(on) to be significantly smaller than RL. This ensures that only a very small part of the supply voltage will be dropped on the MOSFET. The power converted into heat on the MOSFET remains low. The component does not heat up so much that it is destroyed.

There are parameters that describe the electrical properties of a MOSFET. They are specified by the manufacturing process. All parameters are indicated on the manufacturer's data sheet.

UBRBreakdown voltage of the drain-source path.
IDmaxMaximum current that may flow through the drain-source path.
RDS(on) - Resistance of the drain-source path when interconnected.
UGSthThreshold voltage above which the drain-source path will be conductive.
PtotMaximum power dissipation converted into heat in the semiconductor.

The parameters indicated are sufficient to draft circuits with MOSFETs in power electronics. The range in which a MOSFET works is limited by the parameters. 
The following apply...
… no voltages higher than UBR can be switched.
… no currents higher than IDmax can flow.
… no power higher than UDS x ID can be implemented.
… no currents greater than UDS / RDS(on) can flow.


The manufacturer publishes some properties in the form of characteristic curves. The transmission or transfer curve is one of these characteristic curves (Figure 27). It indicates the influence that the gate voltage UGS has on the drain current ID hat. Typically, the threshold current UT must be exceeded for drain current to flow.

We use the characteristic curve to find out how high the gate voltage must be to reliably switch the desired current via the load.

The MOSFET used in the experiments has a particularly low threshold voltage. It is designed for direct usage in logic circuits. These work at 3 to 5 volts.
Another curve is the characteristic output curve (Figure). It indicates to what extent the drain current ID is dependent on the drain-source current UDS for multiple gate voltages UGS. 

The area to the left of the blue line is interesting in switching operation. The resistance of the drain-source path is most efficient in this area.


The flip flop

Task 17

The flip flop, or bistable multivibrator, is an important basic electronic circuit. It generally represents a storage location for a bit. Thousands of these circuits are used in microprocessors, and they are the basic circuit for dynamic RAM modules. The flip flop is also known as a flip flop multivibrator.

Today, flip flops are only attached to individual transistors in exceptional cases. There are a large range of integrated circuits that contain one or more of these switches.

The following logic table shows the correlations between signals on the inputs and the two outputs. This is the simplest form of flip flop, the RS flip flop. The name is derived from the two inputs.















Last stored state










Only as long as signal is applied, cannot be saved

The flip flop frequently has two outputs. The second output is then negated. This means that the opposite logical signal is always present. The negation is indicated by the small circle.

If one of the two inputs is activated with a logical signal, this is transferred to the output and saved. If there are no signals on the inputs, this state is retained. Only a signal on input R can reset the output. If both inputs are occupied with signals, this is an undesirable state. After the signals are removed, the output will remain set or reset depending on the shutdown sequence.

The monostable flip flop

Task 18

The monostable flip flop or monostable multivibrator is a basic electronic circuit with a time response. The time response is illustrated by the square pulse in the wiring diagram (Figure 30). In addition, the wiring diagram also shows that the input does not require a voltage level, but is instead switched by a pulse from 0 to 1. This means that the transition from 0 (no voltage) to 1 (voltage applied) triggers the switching process. The transition is called an edge. This is called a dynamic input. The triangle on the wiring diagram stands for a dynamic input. The monostable flip flop is also called a monostable multivibrator.

The monostable flip flop frequently has two outputs. The second output is then negated. This means that the opposite logical signal is always present. The negation is indicated by the small circle.

If the input is activated with a logical signal, this is transferred to the output and saved. The output automatically switches back to its initial state after a specified time.
The time period can be stipulated by an RC element. An RC element is an interconnected resistor (R) and capacitor (C). The resistor is used to charge or discharge a capacitor. A period of time is required for the process as calculated by the following formula.


The logarithm In(2) can be replaced by the constant 0.7.


Astable flip flop

Task 19

The astable flip flop or astable multivibrator is a basic electronic circuit that switches periodically and automatically between two states. The switching times are defined by 2 RC elements.

There is no specific symbol for an astable flip flop. It is, in principle, a generator with a certain frequency. Since the output voltage moves between two values, it is similar to a rectangle generator.

A certain length of time T is required to complete a full cycle. This is the total of the times for the two RC elements. If both RC elements have the same dimensions, they are built symmetrically. In this case, the following formula can be used. The constant 2 indicates that there are 2 identical RC elements present. 


In practice, the period duration T is rarely indicated. The number of pulses output per time unit is more important. The number of pulses per second is the frequency f. The unit of measure for frequency is the Hertz (Hz). Frequency can be calculated using the following formula:



The Schmitt trigger

Task 20

The Schmitt trigger is a flip flop in which an input voltage is compared to a voltage stipulated in the circuit. One unique feature of the Schmitt trigger is that the activation and deactivation currents are not identical. The difference is called the switching hysteresis or simply the hysteresis.

The output voltage can be close to the supply voltage. A Schmitt trigger is useful for converting any voltage to a fixed logical output level. 

The joint emitter resistance RE is used to rapidly change over the output voltage. A voltage drop once T1 is activated by a base voltage increases the emitter voltage of T2. At the same time, the base voltage of T2 is reduced because T1 is conductive. The voltage between the base and emitter of T2 drops suddenly, T2 is blocked, and the circuit is flipped.

The fact that it delivers clearly defined output levels makes the Schmitt trigger a frequently used circuit type. All signals conducted in the real world are subject to disruptive influences. The transmission of digital information via satellite or long cables under the ocean would be impossible if Schmitt triggers were not able to restore the original signals.

This capability is shown in the Figure. The voltage difference represented by the two black horizontal lines is the hysteresis. The turn-on voltage is always greater than the cut-off voltage.


The differential amplifier

Task 21

A differential amplifier is an amplifier switch with two inputs. The voltage difference between the two inputs is amplified. Differential amplifiers are generally only offered as complete circuits, in the form of integrated circuits. Operational amplifiers are highly important. They consist of a differential amplifier in the input circuit. However, many additional transistor levels are added to this to achieve the desired properties.

Differential amplifiers are very important as operational amplifiers, and in measurement technology.

A simple differential amplifier consists of two transistor amplifiers connected via a joint emitter resistor. The emitter resistor should allow a constant current to flow, independent of the activation of the transistors. So-called constant current sources are integrated into the circuits in industrial applications.