In our everyday lives, we use a lot of electronic devices that contain amplifiers. These amplifiers are used to boost the signals they process, and their basic structure is shown in Fig.1

As shown in the figure, signals output from devices like microphones, tape recorders, or other sensors are usually very weak. This means they can't directly drive loads like speakers. So, we need to use amplifier circuits to boost these small signals to a sufficient amplitude or power level to drive speakers and other loads.

In amplifiers, circuits that handle small voltage or current amplitudes are called small-signal amplifiers. Those that boost signals for things like speakers or relays are called large-signal or power amplifiers. Fig.2 shows a block diagram of the amplification circuit in home audio equipment. The signal source is a tape recorder, and since its output signal is quite small, a preamplifier is used to boost it first. This preamplifier is a small-signal amplifier that amplifies the signal to about 1V. Then, the 1V signal is sent to the main amplifier for power amplification. The main amplifier's output has enough power to drive the speakers, so it's a power amplifier. The DC power supplies in the diagram provide biasing energy for the two stages of amplification.

Amplifiers can be categorized by the frequency of the signals they process: DC amplifiers, low-frequency amplifiers, high-frequency amplifiers, and ultra-high-frequency amplifiers. It's important to note that the DC amplifiers mentioned here aren't for the theoretical DC signals with zero voltage and current change, which can't actually be amplified. In real life, DC signals might be slowly changing voltages that appear constant in the short term but vary over longer periods. DC amplifiers can boost these slow-changing signals without being limited by frequency response. For a classification of amplifier types used for different frequencies, see Fig.3.

DC amplifiers handle signals that include DC components and can usually process AC signals too. Low-frequency amplifiers typically deal with audio signals and are commonly used in audio products.

By power classification, amplifiers with an output power below 10mW are called small-signal amplifiers. Those with output power above 10mW are called large-signal or power amplifiers. For example, to play the signal received by a radio through a speaker, the signal first needs to be amplified to several hundred milliwatts before it can be sent to the speaker for playback.

Small-signal amplifiers are usually voltage amplifiers, which boost the signal voltage with a very small output current, meaning they output low power. These amplifiers have high output impedance, so they can't directly drive loads like speakers, which typically have low impedance, usually only 4 to 8Ω. Fig.5 series shows a diagram of a microphone amplification system. The microphone signal amplifier's output needs to be sent to a power amplifier before it can drive a speaker. If you connect the microphone amplifier directly to the speaker, the signal voltage at the speaker will be very low, preventing the sound from spreading effectively. The equivalent circuit is shown in Fig.5(a).

Fig.5(b) shows a diagram where a power amplifier is added as the second-stage amplifier (with the microphone amplifier as the first stage). The microphone's output impedance is about 1kΩ, which is equivalent to the output internal resistance of the microphone amplifier. The power amplifier's input impedance acts as a load for the microphone amplifier. When the power amplifier's input impedance is 1kΩ, matching the microphone amplifier's output impedance, the input voltage to the power amplifier is half the output voltage due to voltage division. This setup is illustrated in Fig.5(c)

A power amplifier is a current amplifier, and unlike high-output-impedance voltage amplifiers, it has a low output impedance. For example, in Fig.5(c), the power amplifier's output impedance is 8Ω, matching the speaker's impedance of 8Ω. When the amplifier is connected to the speaker, the output voltage is half of what it would be without a load because the power amplifier's output impedance is parallel to the speaker's impedance. This results in a higher output current, providing enough power to drive the speaker.

In an amplifier circuit, the output impedance of the amplifier needs to match the impedance of the load. The load can be something like a speaker or the next stage of amplification. Impedance matching means the output impedance of the amplifier should be equal to or close to the load's impedance. If there's a big difference between them, you'll get impedance mismatch, which leads to serious signal loss and poor amplification. Only with impedance matching can you achieve optimal power transfer.

When we learn about BJT (bipolar junction transistors), we discover that they have more roles than just switching; they can amplify signals too. Because of their cost-effectiveness, BJTs dominate the amplifier market. Field-effect transistors (FETs) also amplify signals but they’re more expensive, so they aren’t as widely used in fields where high precision isn’t required. The most common type of BJT amplifier we see is the Common Emitter Amplifier, which amplifies both voltage and current. This might make students wonder that why we need common base and common collector amplifiers if the common emitter circuit already handles voltage and current amplification?

The common emitter amplifier can't meet all scenarios, so we need common base and common collector amplifiers for different situations. For example, the first stage of an FM radio uses a common base amplifier to amplify the high-frequency radio signals received by the antenna before passing them to the next power amplifier stage to drive the speaker. This is because the common base amplifier has high bandwidth and low input impedance. While the common collector amplifier also has high bandwidth, it has high input impedance, which would block most of the weak signals received from the antenna.

Whether it's an NPN transistor or a PNP transistor, the smallest amplifier circuits they form usually come in three types of configurations: common-emitter amplifier, common-base amplifier, and common-collector amplifier. Each type of circuit has its unique characteristics. As NPN transistors are more commonly used in these minimal amplifier circuits, the comparison between these three configurations will be based on amplifier circuits composed of NPN transistors.

Common Base Amplifier

The input of a common base amplifier is at the emitter and the output is at the collector. For AC signals, the base acts as the common point for both input and output, hence it's called a common base configuration.

Why common base amplifier is low input impedance?

When the input signal enters the emitter, it directly affects the base-emitter voltage, creating a strong feedback effect. This feedback reduces the overall input impedance because any small changes in the emitter voltage are countered by larger variations in the base current.

its input impedance $R_{\mathrm{in}}=\frac{V_{\mathrm{in}}}{I_{\mathrm{in}}}=\frac{-I_b .r_{be}}{-(1+\beta ).I_b}=\frac{r_{be}}{1+\beta }$

Where V_in is the input signal voltage, I_in is the input signal current, I_b is the is the base current, r_be is the resistance between base and emitter, β is the current gain for the common emitter configuration which is a constant given by the transistor manufacture.

Why common base amplifier is high output impedance?

The output is taken from the collector, where the transistor operates with a high voltage gain. This setup means the changes in the input signal cause significant changes in the collector voltage, but the current change is relatively small, according to the Ohm's Law, $R_{out}=\frac{V_{out}}{I_{out}}$, leading to a high output impedance.

Why common base amplifier is High Bandwidth?

Their low input impedance reduces the impact of parasitic capacitances, resulting in faster signal response and higher frequency capability. By limiting voltage variations at the input, low impedance minimizes the influence of these capacitances. This reduction in voltage swing means parasitic capacitances have less effect on the signal, improving bandwidth and signal integrity. Essentially, a lower impedance path allows the signal to travel more easily, thus widening the bandwidth.

Common base amplifier voltage gain, Av:

$A_V=\frac{V_{out}}{V_{\mathrm{in}}}=\beta \frac{{R^{\text{'}}}_L}{r_{be}}$

As R'_L is the parallel resistance of R_C and R_L , and R_C << R_L , so $A_V=\beta \frac{R_C}{r_{be}}$

Where Vout is the output voltage, V_in is the input signal voltage, r_be is the resistance between base and emitter, β is the current gain for the common emitter configuration which is a constant given by the transistor manufacture.

As we can see from the above equation that the voltage gain Av does not has a '-' symbol, so the input signal and the output signal are "in-phase" .

Common base amplifier current gain, Ai:

As I_in=-I_e, I_o=I_c.

So, $A_i=\frac{I_o}{I_{\mathrm{in}}}=\frac{-I_c}{I_e}=-\alpha$

Where I_in is the input current, I_e is the emitter current, I_o is the output current, I_c is the collector current.

As we know, in NPN transistor, the I_c is almost equal to Ie, so, the current gain of a common base amplifier is approximately 1. That is why the common base amplifier does not amplify current.

Power Gain , A_p = Voltage Gain (A_v) × Current Gain (A_i). As A_i is almost 1, so common base configuration power gain A_p=A_v

Common Emitter Amplifier

The input of a common emitter amplifier is at the base, and the output is at the collector. For AC signals, the emitter is connected to the ground of the input and output signals, making it the common point. That's why it's called a common emitter.

The current gain, represented by the Greek symbol Beta (β), is a constant value that shows the amplification factor of the transistor in its active region. You can usually find this value in the datasheet provided by the manufacturer. It can also be measured and calculated in practice using the formula $\beta =\frac{\Delta I_c}{\Delta I_b}$.

The ratio of I_c to I_e is represented by a Greek symbol, α, which measures the efficiency of current transfer in a transistor, describing how effectively the emitter current is transferred to the collector.

$\alpha =\frac{I_c}{I_e}$

As I_e=I_c+I_b .So, Alpha is typically less than 1, but very close to it, usually in the range of 0.95 to 0.99.

Common emitter configuration voltage gain, Av:

$A_v=\frac{V_{\mathrm{out}}}{V_{\mathrm{in}}}=-\frac{\beta {R^{\text{'}}}_L}{r_{be}+(1+\beta )R_e}$

As (1+β)R_e >> r_be, so $A_v\approx -\frac{{R^{\text{'}}}_L}{R_e}$

R'_L is the parallel resistance of R_c and R_L, and R_L >> R_c. So, $A_v\approx -\frac{R_c}{R_e}$. The negative sign shows that the input and output are "out-of-phase", with a phase shift of $180°$.

From the above, we can see that the common emitter voltage gain has nothing to do with β or the bipolar transistor's parameters such as r_be, and so on. It is only determined by the ratio of the resistor connected to the collector R_c and the resistor connected to the emitter, R_e.

Power Gain , A_p = Voltage Gain (A_v) x Current Gain (A_i)

As A_i is β, so common emitter configuration power gain $A_p=\beta .A_v$

Input impedance $R_{in}=\frac{V_{\mathrm{in}}}{I_{\mathrm{in}}}=[r_{be}+(1+\beta \left)\right]//R_b$

Where r_be is the resistance between base and emitter, R_e is the resistor connected to emitter, R_b is the resistor connected to base. As we can see, introducing R_e increases the input impedance compared to without R_e.

Output impedance $R_{out}=\frac{V_{out}}{I_{out}}=R_c//(r_{ce}+{R^{\text{'}}}_e+\frac{\beta .r_{ce}}{r_{be}}.{R^{\text{'}}}_e)$

Where R_c is the resistor connected to collector, r_ce is the resistance between collector and emitter, R^'_e is the parallel resistance of Re and r_be.

A common emitter amplifier has lower bandwidth compared to a common base amplifier because it has high input impedance. This high input impedance, combined with parasitic capacitance in the input signal path, forms an RC filter circuit. As a result, the bandwidth of a common emitter amplifier is lower than that of a common base amplifier.

Common Collector Amplifier

For a common collector amplifier (CC), the input is at the base, and the output is at the emitter. For AC signals, the collector serves as the common point for both input and output, hence it's called a common collector configuration.

The CC configuration is structurally similar to the common emitter (CE) configuration, but it has very different characteristics. CE has high voltage gain, whereas CC's voltage gain is only around 0.9 to 0.99. CE has high output impedance, but CC has low output impedance.

Common Collector Voltage Gain, Av:

$A_v=\frac{V_{out}}{V_{in}}=\frac{(1+\beta ).{R^{\text{'}}}_e}{r_{be}+(1+\beta ).{R^{\text{'}}}_e}$
The gain between input and output is positive, meaning the input and output are "in-phase". Here, ${R^{\text{'}}}_e=R_e//R_L$

From the formula above, we can see that when the transistor operates in the amplification region, as r_be is very small, the voltage gain of the common collector, $A_v\approx \frac{(1+\beta ).{R^{\text{'}}}_e}{(1+\beta ).{R^{\text{'}}}_e}=1$, which gives it the function of a voltage buffer. Here, V_out is the output voltage, V_in is the input voltage, and r_be is the resistance between the base and emitter.

Common Collector Current Gain, Ai:

$A_i=\frac{I_{out}}{I_{in}}=-\frac{I_e}{I_b}=-\frac{(I_c+I_b)}{I_b}=-(1+\beta )$

From the above we can see that the common collector amplifier is commonly known as a Voltage Follower or Emitter Follower. It is called an emitter follower (or voltage follower) because its output voltage 'follows' the input voltage. Simply put, the input signal is applied to the base, and the output signal is taken from the emitter. Since the emitter voltage is almost equal to the base voltage, the output 'follows' the input signal. In this way, the emitter follower provides good current gain without altering the voltage. The term 'follower' is used due to this following characteristic.

Common Collector Input Impedance, R_i:

$R_{in}=\frac{V_{in}}{I_{in}}=r_{be}+(1+\beta ).{R^{\text{'}}}_e$

Here ${R^{\text{'}}}_e=R_e//r_{be}$

From the formula, we can see that the input impedance of the common collector is the sum of the resistance between the base and emitter (r_be) and (1+β)R'_e. This is equivalent to reflecting R'_e into the base loop, which increases the input impedance.

Common Collector Output Impedance, r_out:

As $V_{out}=-I_b(r_{be}+R_s)$ and $I_{out}=-I_e=-(1+\beta ).I_b$

So, $R_{out}=\frac{V_{out}}{I_{out}}=\frac{-I_b(r_{be}+R_s)}{-(1+\beta ).I_b}//R_e=\frac{r_{be}+R_s}{1+\beta }//R_e$
R_s is the resistor resistance connected to the input signal in series or the Internal Impedance of the signal source.

From the formula above, we can see that the output impedance of the common collector is low, usually around tens of ohms. Therefore, it has a better load-driving capability compared to the common emitter and common base amplifiers.

The common collector amplifier has high bandwidth. Despite its high input impedance, its low output impedance allows it to effectively drive loads over a wide range of signal bandwidths, thus maintaining a wide frequency response.

Each of the three configurations,common base, common collector,and common emitter has its own unique characteristics. Here's a comparison chart to illustrate their differences: