# Can the exit be larger than the entrance?

## Collector circuit (emitter follower)

The collector circuit is the basic transistor circuit that is most commonly used in practice, even if it doesn't seem like that. The second most common basic circuit is the emitter circuit followed by the rarely used basic circuit.
The collector circuit is also known as an emitter follower. This is because the emitter appears to be following the voltage on the base. The special features of this circuit is a voltage gain of less than 1, but a very large current gain, which is dependent on the differential current gain factor β of the transistor.
The term emitter follower was adopted from tube technology. Here it is the cathode follower circuit. However, with transistors, the equivalent of the cathode of a tube is the emitter.

A simple collector circuit consists of a transistor, the emitter resistor R.E., the basic series resistor RV and the operating voltage UB.. The emitter is the exit. The collector is for the input and output voltage via the operating voltage UB. the common reference point.

### Current and voltage distribution

In the case of the collector circuit, the resistors R1, R2 and RE. the working point. The calculation of the operating point is the same as for the emitter circuit, only without a collector resistor. When setting the operating point, a general distinction is made between small-signal transmission and large-signal transmission.
The emitter resistance RE. is an integral part of the circuit. It always stabilizes the working point in the collector circuit through negative current feedback. In contrast to the emitter circuit, the negative feedback cannot be suppressed in the collector circuit. Here you have to live with the fact that the negative feedback affects both the DC voltages and the signal voltages.
If AC voltage is amplified, the circuit must be connected via the coupling capacitors C.K be connected to the signal source and the load. No direct current flows through the coupling capacitors. The signal source or load therefore has no influence on the operating point. The voltages of the operating point can be selected independently of the DC voltages of the signal source and load.
In the case of the collector circuit, the input voltage is Ue and output voltage Ua in phase.

### Emitter follower as impedance converter (only with resistive load)

The emitter follower is distinguished by two further properties. That is the very large input resistance and the very small output resistance. This is one of the reasons why this circuit is often referred to and also used as an impedance converter.
However, this only applies if the load resistance in the usual transistor circuits is not complex. "Complex" means that the load has an inductive or capacitive component. For example, when operating a loudspeaker, the phase shift cannot be foreseen. Here both the amplitude and the frequency are extremely variable, which destroys the sound of an amplifier.

On the other hand, due to the inductive or capacitive load, the voltage at the output can be greater than the emitter follower is currently delivering. That drives the transistor into the
de-energized state. The complex load is exposed at the emitter terminal. If the voltage is high enough at this moment, the emitter-base current occurs in the opposite direction and the transistor says goodbye.

In the case of alternating voltages to be amplified, it is advisable to work with a push-pull emitter follower in a voltage-symmetrical manner. In the same situation, the two transistors of the push-pull emitter follower supply and sink the current, depending on who is playing the source. Output stage or complex load.

### Small signal transmission

The emitter voltage UE. can be selected small, but must be at least 1 V if the operating point is to be stable. The emitter voltage UE. be greater than the amplitude of the output voltage.

The size of the collector current I.C. depends on the signal size and the required transmission properties. Basically, the collector current IC. be greater than the amplitude of the output current.

### Large signal transmission

In the case of large-signal transmissions, U is selected for the emitter voltageE. usually about half the operating voltage UB..

The collector current IC. depends on the load resistance RL.. The collector current IC. is to be understood as the output current. The performance adjustment often applies here. That means the emitter resistance RE. becomes the same as the load resistance R.L. elected.

### Input resistance re

The collector circuit has a large input resistance re, which is divided by the resistors R1, R2, RE. || R.L. and the alternating current gain forms β.

### Output resistance ra

The collector circuit has a small output resistance ra.

### Voltage gain Vu

The output voltage is always lower than the input voltage. The voltage gain Vu is about 1.

### Current gain Vi

The collector circuit has a high current gain. The power gain is equal to the current gain. The greatest current gain is achieved when the load resistance is the emitter resistance R.E. is.

### Overview: The collector circuit in comparison

circuitEmitter circuitBasic circuitCollector circuit
Input resistance re100 Ω ... 10 kΩ10 Ω ... 100 Ω10 kΩ ... 100 kΩ
Output resistance ra1 kΩ ... 10 kΩ10 kΩ ... 100 kΩ10 Ω ... 100 Ω
Voltage gain Vu20 ... 100 times100 ... 1000 times<=1
DC gain B 10 ... 50 times<=110 ... 4000 times
Phase rotation 180°
Temperature dependence largesmallsmall
Power amplification Vpvery largemediumsmall
Cutoff frequency fGlowhighlow
Applications LF and HF amplifiers
Power amplifier
counter
Impedance converter

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