Transistor Configurations

Transistors have three terminals connected to input and output circuit loops. One-transistor amplifiers are two-port networks; one of the three transistor terminals must be shared by both input and output ports as the common terminal. This results in three possibilities. The first is the common emitter (CE) amplifier. The emitter is common to both input and output, as shown below.

The emitter is part of both input and output loops. It is the common terminal of the transistor that is connected to both an input and output port terminal. With a series emitter resistor RE the emitter terminal is still common to both loops. The output loop current is shown flowing from the power supply (+VCC), through RL and the BJT, through RE to ground, which is connected to the negative terminal of the supply. The closure of the output loop from ground to +VCC implies flow through the voltage-source, +VCC.

The common-base (CB) configuration is shown below:

The common-collector (CE) configuration, also known as the emitter-follower, is shown below.

Common-Emitter Amplifier

The voltage gain was found by the transresistance approach: a ratio of output (load) resistance and transresistance, the resistance across which the input voltage develops the common (emitter) current. Not all of the emitter current gets to the collector. Some is lost to the base, and the a factor accounts for this in the voltage-gain equation:

Because a >>1, the voltage gain is a ratio of resistances. The input voltage vi is applied across rM, producing iE = vi/rM. Then iC ( = a x iE) gets through to the collector and develops a voltage of vo = - iC x RL at the output. By solving these equations for Av, the above gain equation results.
The input resistance of the CE is vi/ii = vi/iB or

The resistance of the input loop is the base resistance in series with the resistance in the emitter-side of the circuit, referred to the base by the b transform.
At the output node, the BJT transistor model shows a current source (infinite resistance) in parallel with load resistance RL. The output resistance is therefore RL.
The CE amplifier has relatively high input resistance due to the b-transform effect at the base. It is better as a voltage-input port. Its output resistance is relatively low if the load resistor is not made too large.
The current gain of the CE is io/ii = iC/iB = b. Its input-loop transresistance used to calculate gain is rM, but the overall amplifier transresistance is Rm = vo/ii = Av x rin and its transconductance is the inverse of the transresistance, or Gm = 1/Rm.

Common-Base Amplifier

The CB amplifier input source is in the emitter loop so that emitter current flows through it. This current is b + 1 times larger than the base current. Consequently, the CB input resistance is relatively low and would make a better current-input than voltage-input port. Its input resistance is

or typically about RE. Its output resistance is the same as the CE, or RL. The CB voltage gain is

Unlike the CE, it is non-inverting (no negative sign). The CB current gain is a, or slightly less than one.
Compared to the CE, the CB input resistance is lower by (b + 1) and is therefore better as a current-input port than the CE. According to the ideal-port table, the CB most closely approaches an ideal current amplifier, though its current gain is slightly less than one!

Common-Collector Amplifier

The CC or emitter-follower has the same input resistance as the CE but its output resistance is

or typically about re, a relatively small resistance of around a few ohms. With high input resistance and low output resistance, the CC appears to approach the ideal voltage amplifier. Unfortunately, its voltage gain is only

or typically somewhat less than one. The port resistances approach the ideal but the voltage gain is not high enough to be useful. The current gain, however, is b + 1.
None of the three single-transistor configurations is ideal as any of the four amplifier types. Amplifiers can better approach the ideal by combining configurations into multi-transistor amplifiers.

Cascade Amplifier

Amplifiers are cascaded when the output of the first is the input to the second. The combined gain is

where vi2 = vo1. The total gain is the product of the cascaded amplifier stages.
The complication in calculating the gain of cascaded stages is the non-ideal coupling between stages due to loading. Two cascaded CE stages are shown below.

Because the input resistance of the second stage forms a voltage divider with the output resistance of the first stage, the total gain is not the product of the individual (separated) stages.
The total voltage gain can be calculated in either of two ways. First way: the gain of the first stage is calculated including the loading of ri2. Then the second-stage gain is calculated from the output of the first stage. Because the loading (output divider) was accounted for in the first-stage gain, the second-stage gain input quantity is the Q2 base voltage, vB2 = vo1.
Second way: the first-stage gain is found by disconnecting the input of the second stage, thereby eliminating output loading. Then the Thevenin-equivalent output of the first stage is connected to the input of the second stage and its gain is calculated, including the input divider formed by the first-stage output resistance and second-stage input resistance. In this case, the first-stage gain output quantity is the Thevenin-equivalent voltage, not the actual collector voltage of the stage-connected amplifier. The second way includes interstage loading as an input divider in the gain of the second stage while the first way includes it as an output divider in the gain of the first stage.
By cascading a CE stage followed by an emitter-follower (CC) stage, a good voltage amplifier results. The CE input resistance is high and CC output resistance is low. The CC contributes no increase in voltage gain but provides a near voltage-source (low resistance) output so that the gain is nearly independent of load resistance. The high input resistance of the CE stage makes the input voltage nearly independent of input-source resistance. Multiple CE stages can be cascaded and CC stages inserted between them to reduce attenuation due to inter-stage loading.

Darlington Amplifier

A CC stage followed by another CC stage has an input resistance of about (b + 1)2 times the emitter resistance of the second stage. More precisely, using the b transform, it is

If RE1 is removed, the second term is about b2 times RE2. Furthermore, if the collectors are connected together, the result is a Darlington stage, as shown below

This stage can be viewed as a "Darlington transistor" because it has three terminals and an equivalent b of about b2. Darlington BJTs can be used in any of the three BJT configurations.
Differential Amplifier
A differential or emitter-coupled BJT pair is formed, as shown below, by a CC/CE stage driving a CB stage. The first stage is a CE to the first output, vo- and is a CC to the second stage.

A differential-input amplifier has an input port for which the negative (- ) terminal is not necessarily connected to the common node (usually ground). A differential amplifier (or diff-amp) amplifies the difference between its input terminals:

Amplifiers with differential outputs have two output terminals, neither of which is necessarily common with an input terminal or ground. The output is
xo = xo+ - xo-
The 2-transistor diff-amp has differential inputs and outputs. The voltage gain is found by calculating the gain from each input to each output (4 gains). The differential gain is the ratio of the difference of the outputs over the difference of the inputs. If the gain magnitude (absolute value, neglecting sign) to the output is different for the two inputs, the amplifier is not differential.
The above amplifier gain can be calculated using the transresistance method. The current-source resistor REE forms a divider between stages. Ideally, REE is a current source. The diff-amp circuit is also symmetrical if corresponding components have equal values:

RL1 = RL2 = RL
RE1 = RE2 = RE
RB1 = RB2 = RB
and
REE >> RE
then the voltage gain is

For non-negligible REE, a divider is formed between stages consisting of the source-transistor RE and REE. Apply Thevenin’s theorem for a Thevenin equivalent source driving RE of the other stage.