# What is a coupling capacitor

## Emitter circuit

The emitter circuit is a universal amplifier circuit that is used in the low-frequency range (NF) to generate very high voltage amplifications. But at high frequencies the frequency dependence of the circuit becomes noticeable. If the frequency increases, the gain decreases. For this reason, the emitter circuit is only operated with a small voltage gain. Because the transistor is temperature-dependent and the operating point changes with the temperature, the emitter circuit is operated with operating point stabilization through current negative feedback.
The emitter circuit is also often used to control push-pull output stages.

Note: This emitter circuit is the simplest and also the worst amplifier circuit.

### Basic circuit

The emitter circuit essentially consists of a transistor, the collector resistor R.C., the input signal source Ue with the basic series resistor RV or a voltage divider (R1 and R2) and the operating voltage + UB.. The collector connection of the transistor is the output for the output voltage U.a. The emitter connection is the common reference point for input and output voltage. This is why this circuit is called an emitter circuit.

Note on the working point: You can do this by choosing RC. and RV optimize in function of the current gain (specimen variance and temperature-sensitive) when the ambient temperature is stable. The collector-emitter voltage depends on it.

### Current and voltage distribution

The alternating voltage Ue is via the coupling capacitor C.K created. Via the voltage divider R1 and R2 the operating point is set. As a result, the base-emitter voltage UBE Set to 0.3 V (germanium) or 0.6 V (silicon) depending on the transistor.
The resistance RC. is significantly involved in the maximum voltage gain. And it limits the collector current I.C. for the transistor.
The coupling capacitors CK separate the AC signal from the DC voltage. The amplified signal is via a further coupling capacitor C.K as alternating voltage Ua issued. It is important to note that the phase of the input and output voltage is rotated by 180 °.
Important, input and output voltage are not out of phaseas it is written in other literature on emitter switching. This phase shift of 180 ° is an inversion or inversion. A phase shift only occurs at higher frequencies when the Miller effect comes into play (frequency-dependent phase shift).

### Function of the coupling capacitors CK

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.
The coupling capacitor CK at the output forms a high pass with the subsequent load resistor. The coupling capacitor CK at the input forms with the input resistance of the amplifier circuit, which is mainly made up of the parallel resistance value from R.1 and R2 results in a high pass.
The coupling capacitors must be dimensioned in such a way that the lowest frequency of the signal to be transmitted still comes through the high-pass filter. DC voltages (0 Hz) do not get through.

### Formula for calculating the DC gain B

The emitter circuit amplifies the direct current component of the input voltage Ue. The DC gain is 10 ... 50.

### AC gain ß

Often the DC gain and AC gain are similar. Therefore, only the DC gain is given in the transistor data sheets.

### Working point setting for the emitter circuit

In order for the emitter circuit to function properly, the voltage and current values ​​must be set correctly. The collector and base current values ​​of the transistor must be taken into account. A general distinction is made between two options for setting the operating point.

### Working point stabilization in the emitter circuit

All transistor values ​​are temperature dependent. This means that the operating point of the transistor is also temperature-dependent. Depending on the application of the transistor and the place of operation, the temperature can act on the emitter circuit and shift the operating point. Shifting the operating point leads to non-linear distortions at the output of the emitter circuit.
As a rule, the collector current I increases as the temperature risesC. to. To counteract this, the base-emitter voltage U is reducedBE and thus prevents the collector current I from risingC.. The difficulty is to determine the base-emitter voltage UBE to reduce or enlarge so that an operating point stabilization occurs.
When stabilizing the operating point, a distinction is made between temperature compensation and negative feedback.

The operating point becomes more stable when the negative current feedback comes into play with an emitter resistor. A control process takes place, which leads to the fact that, among other things, the non-linear distortion is reduced.

### Overview: The emitter 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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