Basic transistor circuits

The basic circuits of an amplifier stage are named after the electrode , which is at a firmly defined electrical potential . This is the electrode that the input and output circuit have in common. In the case of a bipolar transistor with its three electrodes, emitter, collector and base , the emitter circuit , the collector circuit and the base circuit result . Due to its properties, the collector circuit is usually called an emitter follower . The basic transistor circuits differ fundamentally in their electrical properties and therefore in their intended use.

Overview

The arrangement of a large number of basic electronic components contained in a device such as an audio amplifier can be (at least conceptually) divided into dozens of the basic circuits described here. The overall function results from the combination and interaction of the individual basic circuits.

The basic circuits with bipolar transistors are described in more detail below. Instead of using bipolar transistors, the analog circuits described can also be implemented with field effect transistors (FET) or electron tubes . The properties of the corresponding circuits are not identical, but their behavior is similar because of the same underlying principles. The corresponding FET circuits are called source circuit, drain circuit / source follower and gate circuit , while the tube circuits analogous to them are called cathode base circuit , cathode follower / anode base circuit and grid base circuit .

The circuits are usually shown in the top row, as shown in the following picture, to illustrate the common electrode in each case. How it works becomes clearer, however, if you redraw the circuits according to the bottom row.

Basic transistor circuits (emitter, collector, base basic circuit)

The above-mentioned method for determining the respective basic circuit is not always strictly met, so that a further criterion must be applied: The designation of the basic circuit is based on the electrode of the transistor at which the common reference potential of the input and output is located. Or: The designation is based on the connection of the transistor, which serves neither as an input nor as an output of the circuit.

Correspondence of the basic circuits including English-language equivalents

Bipolar transistor	Emitter circuit common emitter	Collector circuit (emitter follower) common collector (emitter follower)	Basic circuit common base
FET	Source switching common source	Drain circuit (source follower) common drain (source follower)	Gate circuit common gate
Electron tube	Cathode base circuit common cathode	Anode base circuit ( cathode follower ) common plate (cathode follower)	Basic grid circuit common grid

Emitter circuit

The emitter circuit is based on the basic function of the bipolar transistor: An alternating signal current flowing into the base causes an alternating current that is greater than the alternating current amplification factor in the collector. ${\displaystyle \beta }$

Amplifier stage in common emitter circuit with operating point stabilization through DC negative feedback

Low noise emitter circuit with voltage feedback ( current mirror as a current source)

The figure opposite shows an amplifier stage for alternating voltage in emitter circuit with capacitively bridged emitter resistance. With the resistors and is working set. In addition, an operating point stabilization can be seen here (see section below) through direct current negative feedback by means of a resistor . The capacitors determine the lower limit frequency of the circuit. They are so large that they can be viewed as a short circuit for the useful alternating current signal to be amplified. connects the emitter to ground in terms of alternating current. and block the DC voltage components at the input and output. The base current controls the collector-emitter current, which is greater by the AC amplification factor. ${\displaystyle R_{1}}$ ${\displaystyle R_{2}}$ ${\displaystyle R_{4}}$ ${\displaystyle C_{3}}$ ${\displaystyle C_{1}}$ ${\displaystyle C_{2}}$ ${\displaystyle \beta }$

The input resistance is small and the parallel circuit complies with , and the base-emitter resistance . If it is omitted, the input resistance increases, because the resistance is then included in the calculation. The output resistance is the parallel connection of the working resistance and the collector-emitter resistance (this is usually very large compared to ). If there is no voltage gain, it is the ratio of to , otherwise it depends on the transistor type and the temperature. The emitter current is equal to the collector current plus the base current. ${\displaystyle R_{1}}$ ${\displaystyle R_{2}}$ ${\displaystyle r_{\mathrm {BE} }}$ ${\displaystyle C_{3}}$ ${\displaystyle r_{\mathrm {BE} }}$ ${\displaystyle r_{\mathrm {BE} }+\beta \cdot R_{4}}$ ${\displaystyle R_{3}}$ ${\displaystyle r_{\mathrm {CE} }}$ ${\displaystyle R_{3}}$ ${\displaystyle C_{3}}$ ${\displaystyle R_{3}}$ ${\displaystyle R_{4}}$

The disadvantage is the reduction in the cut-off frequency due to the Miller effect . This can be avoided by using a cascode made up of two transistors.

The basic circuit with field effect transistors , which is analogous to the emitter circuit , is referred to as a source circuit ; the corresponding basic circuit with electron tubes is called the basic cathode circuit.

Dimensioning of the components

The voltage on should be around 0.6 V, i.e. the voltage on 1.2 V. should be so high that the voltage on the collector is exactly half as high as the operating voltage , because then both half-waves can reach their maximum value. The circuit in Figure 2 has no AC negative feedback and therefore distorts the signal. This can be significantly improved by adding a small resistance of around 100 Ω in series . However, this also reduces the gain. ${\displaystyle R_{4}}$ ${\displaystyle R_{2}}$ ${\displaystyle R_{3}}$ ${\displaystyle U_{\mathrm {cc} }}$ ${\displaystyle C_{3}}$

properties

• inverting
• Current gain high
• Voltage gain high
• Power gain approx. 100–1000, approximately voltage gain × current gain
• Input resistance: 500 Ω – 2 kΩ
• Output resistance: 50 Ω – 100 kΩ or roughly the same as the working resistance R ₃
• Low-distortion amplification only for very small input voltages: if C _{3 is} present <0.001 V, otherwise depending on the ratio ${\displaystyle {\frac {R_{3}}{R_{4}}}}$

Areas of application

The emitter circuit is used in many areas of electronics, for example in small-signal amplifiers and electronic switches. It is by far the most common of the three basic circuits.

Stabilization of the working point

DC negative feedback

→ Main article : Working point

The type of stabilization of the operating point is in principle independent of the basic transistor circuit. A distinction is made between the following stabilization circuits:

• Stabilization through emitter resistance or DC negative feedback (see figure " DC negative feedback ")

  The transistor heats up during operation, which makes it more conductive and a larger collector current flows. The larger collector current causes a larger voltage drop across the emitter resistor . The base-emitter voltage decreases and the transistor blocks more. ${\displaystyle R_{4}}$

• DC negative feedback (see adjacent figure)

  If the collector current increases due to self-heating of the transistor, more voltage drops across the resistor . This makes the base-emitter voltage and the collector-emitter voltage smaller. The transistor blocks more and the collector current becomes smaller. ${\displaystyle R_{\mathrm {c} }}$

Collector circuit (emitter follower)

Collector amplifier

The supply voltage source should not have any resistance for the signal (if necessary, connect a capacitor in parallel), so the collector is at a constant voltage level. A small base-emitter current flows in the circuit and controls a larger collector-emitter current. This is determined by the work resistance; It has a voltage with the input voltage and the base-emitter voltage of approximately 0.7 V. ${\displaystyle R_{3}}$ ${\displaystyle U_{\mathrm {a} }=U_{\mathrm {e} }-U_{\mathrm {be} }}$ ${\displaystyle U_{\mathrm {e} }}$ ${\displaystyle U_{\mathrm {be} }}$

Collector circuit as an ideal transistor through impedance converter via operational amplifier , can also be seen as an amplified voltage follower (emitter follower) and is the basic circuit of linear voltage regulators : U _e = U _a

The output voltage at the emitter therefore approximately follows the input voltage, which is why one speaks of an emitter follower circuit . Since the current through the load resistance at the input appears to be reduced by the factor of the current gain, the input impedance of an emitter follower circuit is very high, the voltage gain is around 1. This makes the circuit an impedance converter .

The analog basic circuit with field effect transistors is referred to as a drain circuit or source follower ; the corresponding basic circuit with electron tubes is called a cathode follower or anode base circuit.

Dimensioning of the components

The voltage on should be exactly half as large as the operating voltage , because then both half-waves can reach their maximum value. This is achieved when and are the same size. ${\displaystyle R_{3}}$ ${\displaystyle U_{\mathrm {cc} }}$ ${\displaystyle R_{1}}$ ${\displaystyle R_{2}}$

properties

• Non-inverting
• Voltage gain almost 1
• Current gain high
• Power gain almost equal to current gain
• Large input resistance: 3 kΩ to 1 MΩ (load resistance × current gain)
• Output resistance small: 0.5–30 Ω
• Low-distortion transmission for input voltages up to the supply voltage

Areas of application

Impedance converter , e.g. B. for crystal pickups and piezo sound pickups, in condenser and electret microphones, as a preliminary stage of the Darlington circuit (here the load is the basis of the output stage) and many audio amplifier output stages .

Basic circuit

It corresponds to the emitter circuit, but the base is grounded or a constant voltage and the emitter current must also flow through the signal source. This leads to a current gain of 1. The input resistance is very small, since the entire load current and the base current have to be applied by the source. The output resistance and the voltage gain correspond to those of the emitter circuit.

The analog basic circuit with field effect transistors is referred to as a gate circuit ; the corresponding basic circuit with electron tubes is called a grid-based circuit .

properties

Basic circuit amplifier

• Non-inverting
• Current gain slightly below 1
• Voltage gain high
• Power gain approx. 1000
  • ⇒ voltage amplification
• Voltage gain 5% to 10% greater than with the emitter circuit
• Input resistance small: 1–100 Ω
• Output resistance large: corresponds roughly to the collector resistance
• higher cutoff frequency due to less feedback
• Low-distortion amplification for input voltages up to a tenth of the supply voltage

Areas of application

• HF levels
• HF oscillators from approx. 50 MHz

Combinations

Combinations of the basic circuits result in the following circuits:

• Parallel connection: several transistors are connected in parallel, but with bipolar transistors each requires its own emitter resistor to ensure the current distribution (not required for MOSFET and IGBT )
• Cascade connection; Series connection of several transistors in emitter connection, the blocking voltages add up, each transistor requires its own, electrically isolated base control
• Cascode : An emitter circuit (below) with a basic circuit above it results in a cascode amplifier in which the input resistance is low and the output resistance is very high. This circuit has particularly low feedback and is therefore suitable for HF applications.
• Transistor-transistor-logic- inverter: base circuit with subsequent emitter circuit.
• Darlington pair : two transistors connected one behind the other; the base of the second is the load of the first, they share the voltage between base 1 and emitter 2. The Darlington circuit can be viewed as a single transistor with a high current gain, there are also integrated Darlington circuits, called Darlington transistors, manufactured.
• Thyristor circuit, multivibrator : Two emitter circuits with feedback.
• Schmitt trigger : Two transistors in a collector circuit, but with a common emitter resistor.

In the output stages of TTL technology, two transistors are operated in a half-bridge arrangement, the lower in an emitter circuit, the upper in a collector circuit.

In the case of the current mirror , the second transistor works in emitter circuit, the first provides the voltage at the base of the second so that its collector current equals the input current; Use as a controllable power source.

At the differential input , e.g. B. an operational amplifier , each of the two inputs acts as an emitter circuit (inverting) on ​​the next level assigned to it, but as a result of the collector circuit and base circuit on the other output.

literature

• Hans-Joachim Fischer, Wolfgang E. Schlegel: transistor and circuit technology . Military publishing house of the GDR, Berlin 1988, ISBN 3-327-00362-9 .
• Rainer Funke, Siegfried Liebscher: Basic electronic circuits . Verl. Technik, Berlin 1975.
• Johann Siegl: Circuit technology - analog and mixed analog / digital: development methodology, amplifier technology, functional primitives of circuits . Springer-Verlag, Berlin / Heidelberg 2005, ISBN 978-3-540-27515-2 , doi : 10.1007 / 3-540-27515-0 .
• Stefan Goßner: Basics of electronics (semiconductors, components and circuits) . 11th edition. Shaker, Aachen 2019, ISBN 978-3-8440-6784-2 .

Web links

• Hanspeter Hochreutener: transistor amplifier circuits. (PDF; 871 kB) Center for Signal Processing and Telecommunications, January 14, 2011, accessed on January 8, 2013 . 

Individual evidence

1. ↑ Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . Springer, 2002, ISBN 978-3-540-42849-7 , pp. 98 .
2. ↑ Christoph Schenk, Eberhard Gamm: Semiconductor circuit technology . 15th, revised. and exp. Edition. Springer, Berlin 2016, ISBN 978-3-662-48354-1 , pp. 101 .