In common emitter amplifier the output is picked at the point Vc. R1 is much larger, while R2 is often also present to increase the thermal stability. It amplifies both current and voltage is the most common way to use the transistor. This amplifier inverts the signal (positive drop in the input Vbb result the negative drop in the output Vc). Indeed, the first semiconductor inverters were very similar to the common emitter amplifiers by design. The input impedance is not very big. When the two common emitter amplifiers are connected sequentially, the first amplifies much worse because of the load of the second cascade. Early amplifier designs even had transformers between cascades.
Differently, in common collector amplifier the output is picked at the point Ve. R2 is much larger than R1 (if R1 is present at all). The output that is now taken at different point (Ve) repeats the input without inversion and also without any voltage amplification (hence this circuit is also called emitter follower). However the amplifier has a very high input impedance, amplifying current (rather than a voltage). It is an appropriate cascade to take the signal from some sensitive source that must not be loaded with high currents, also it is possible to put it between two common emitter cascades, creating more optimal work conditions for the first common emitter amplifier.
To understand why the common emitter and common collector circuits have the described features, it is important to know processes that define the work of the device.
Input voltage causes the base current, loading the source. This flow goes from the input Vbb through R3, the base - emitter junction of the transistor then through R2 to the ground. It is a common error for a novice to assume that the input impedance is R3 + R2 + the impedance of the transistor junction (small under normal operation). This would only be true for the totally closed transistor. When the transistor is at least partially open, the collector current that also passes through R2, creating the non zero potential on it. When computing the input current, this potential must be subtracted from the input voltage before dividing by R3 + R1, finally getting much lower values. The emitter follower can have impedance of many megaohms when both R3 and R2 are well below this range. The actual input impedance is equal to
where can be approximately understood as the resistance of the base-emitter junction. is the transistor-specific current gain, explained more in the next paragraph.
Base - emitter current opens the transistor, causing the collector - emitter current. This second current is larger than the base current. The ratio between change of the collector current and base current is called beta coeffcient () or h21e:
This coefficient is usually 5 - 25 for some really bad transistors of old design, somewhere about 50 for a typical transistor of 70's and usually over 100 for a recent transistor. It differs significantly even between the transistors of exactly the same type. Some advanced devices require to measure this coefficient to select the most appropriate instance for the particular circuit. This coefficient however does not tell directly how much can you amplify current or voltage with this transistor: this depends on how R1 and R2 are selected. Interestingly, the input impedance of emitter follower directly depends on the coefficient.
The current through transistor is also limited by R1, R2 and have some maximal value at the given power voltage (Vcc). Near this limit the transistor enters saturation state: further increase of the base current cannot increase the collector current. On the other end, there is some minimal voltage (usually 0.4 to 0.7 V) that must drop on the base-emitter junction to have operational currents through it. When the voltage is below this limit (or zero), the resistance of junction significantly raises, the current drops and the transistor closes - this is called cut-off state. Transistor does not work properly as an amplifier when it is in cut-off or saturation stage. Saturation is undesirable even in digital circuits as logic gate that enters saturation in one of its states is slower.
Changes of the collector current cause voltage changes on R1, the resistor in the collector circuit. Here it is important that this change is equal to the change of the base current multiplied by coefficient; it is much less dependent on the value of R1 itself. However the higher the resistance of R1, the higher voltage change is required to alter the current through it:
If the resistance of R1 is high enough, the voltage changes (at point Vc) can be hundred and more times larger than the initiating changes at point Vbb - transistor works as a voltage amplifier. However really big resistance of R1 makes the amplifier unusable due too high output impedance that is close to the value of this resistor.
Changes of the current emitter - collector current create negative feedback through collector resistor R2. Voltage increase at Vbb increases the current through base-emitter junction, opening the transistor more. When the transistor opens, more current passes through R2 and as a result more voltage falls on it. However this reduces the voltage falling on the base-emitter junction, reducing also the current passing through it - this is the negative feedback. The negative feedback reduces voltage amplification but increases stability of the amplifier: its work is then less dependent on the resistance of the base-emitter junction (that varies significantly with the temperature) and the coefficient of the transistor (that varies in multiple amplifiers as if produced in a factory). Due these useful features R2 is also often present in common emitter amplifiers. If the signal frequency is high enough, the voltage gain can be increased by shunting it with capacitor.
When R2 is present and its resistance is high enough, the voltage gain becomes less dependent on the transistor and instead becomes close to:
Minus in front of equation means that the output is inverted, taking it at Vc point).
Current through the transistor releases some power inside it, creating the heat that must be radiated away. Here it may be somehow contra-intuitive that while fully closed transistor does not release much heat, the fully open transistor does not release a lot of it either. Even high current does not mean a lot of power if the voltage (falling on the transistor) is low. The most of heat is released when the transistor on only half open, as in this case it may be both significant current and the significant voltage falling on the transistor.