Emitter follower Circuit Simulation Using Multisim
Emitter follower is a case of negative current feedback circuit. This is mostly used as a last stage amplifier in signal generator circuits. The important features of Emitter Follower are − It has high input impedance. It has low output impedance.
It is not always possible to directly couple the emitter follower, common collector buffer. When this is the case, it is necessary to add a few additional electronic components: coupling capacitors and bias resistors to the circuit.
The emitter follower can be designed and electronic component values determined using the design flow below as a basis:
1. Choose transistor:
As before, the transistor type should be chosen according to the anticipated performance requirements.
2. Select emitter resistor:
Choosing an emitter voltage of about half the supply voltage to give the most even range before the onset of any clipping, determine the current required from the impedance of the following stage.
3. Determine base current:
The maximum base current is the collector current divided by β (or hfe which is essentially the same).
4. Determine the base voltage:
The base voltage is simply the emitter voltage plus the base emitter junction voltage – this is 0.6 volts for silicon and 0.2 volts for germanium transistors.
5. Determine base resistor values:
Assume a current flowing through the chain R1 + R2 of around ten times that of the base current required. Then select the correct ratio of the resistors to provide the voltage required at the base.
6. Determine value of input capacitor value:
The value of the input capacitor should equal the resistance of the input circuit at the lowest frequency to give a -3dB fall at this frequency. The total impedance of the circuit will be β times R3 plus any resistance external to the circuit, i.e. the source impedance. The external resistance is often ignored as this is likely to not to affect the circuit unduly.
7. Determine output capacitor value:
Again, the output capacitor is generally chosen to equal the circuit resistance at the lowest frequency of operation. The circuit resistance is the emitter follower output resistance plus the resistance of the load, i.e. the circuit following.
8. Re-evaluate assumptions:
In the light of the way the circuit has developed, re-assess any circuit assumptions to ensure they still hold valid. Aspects such as the transistor choice, current consumption values, etc.
The emitter follower circuit is particularly useful for applications where a input high impedance is required. Offering a high input impedance and low output impedance it is does not load circuits that may only have a small output capability, or those circuits like oscillators that need a high impedance load to ensure the optimum stability, etc.