11.3 BJT amplifier configurations

BJT Amplifier Configurations

Every BJT amplifier circuit shares the same three terminals (base, collector, emitter), but which terminal you treat as "common" to both input and output completely changes the amplifier's behavior. The three standard configurations each trade off voltage gain, current gain, and impedance in different ways, so picking the right one depends on what your circuit needs to do.

Common Emitter Configuration

The common emitter (CE) is the most widely used BJT amplifier configuration, and it's the one you'll encounter first in most courses. The emitter terminal is shared between the input and output sides of the circuit.

• Input signal is applied between the base and emitter; output is taken between the collector and emitter
• Provides high voltage gain, high current gain, and therefore high power gain
• Inverts the output signal by 180° relative to the input (this is a classic exam point)
• Moderate input impedance, moderate-to-high output impedance
• Typical use cases: audio amplifiers, general-purpose voltage amplification, signal conditioning stages

Because CE gives you gain in both voltage and current, it delivers the highest power gain of the three basic configurations. The trade-off is that 180° phase inversion, which you need to account for in multi-stage designs.

Common Collector Configuration

The common collector (CC) is also called the emitter follower because the output voltage "follows" the input voltage almost exactly. Here the collector is the terminal common to both input and output.

• Input signal is applied between the base and collector; output is taken between the emitter and collector
• Voltage gain is approximately unity ($A_v \approx 1$), so it doesn't amplify voltage
• High current gain (output current is roughly $\beta$ times the input current)
• No phase inversion between input and output
• Very high input impedance, very low output impedance
• Ideal for impedance matching and buffering: connecting a high-impedance source to a low-impedance load without losing signal

The near-unity voltage gain comes from 100% negative feedback through the emitter resistor. That same feedback gives the emitter follower excellent linearity and low distortion, which is why it shows up in voltage regulators, power supply outputs, and audio output stages.

Common Base Configuration

The common base (CB) is the least common of the three in everyday circuits, but it excels at high frequencies. The base terminal is shared between input and output.

• Input signal is applied between the emitter and base; output is taken between the collector and base
• Current gain is approximately unity ($A_i \approx 1$), but voltage gain can be high
• No phase inversion between input and output
• Low input impedance, high output impedance
• Best suited for high-frequency applications (RF amplifiers) because the grounded base shields the input from output capacitance, reducing unwanted feedback at high frequencies

Amplifier Characteristics

Gain Parameters

Voltage gain ($A_v$) is the ratio of output voltage to input voltage. For a basic common emitter amplifier with an unbypassed emitter resistor:

$A_v \approx -\frac{R_C}{R_E}$

The negative sign reflects the 180° phase inversion. If the emitter resistor is bypassed with a capacitor (common in AC analysis), the gain increases significantly but becomes more dependent on the transistor's small-signal parameters.

Current gain ($A_i$) is the ratio of output current to input current. It depends heavily on the transistor's $\beta$ (DC current gain) and the specific configuration. CE and CC configurations have high current gain; CB has current gain close to 1.

Power gain ($A_p$) combines both:

$A_p = A_v \times A_i$

CE wins here because both $A_v$ and $A_i$ are greater than 1.

Impedance Characteristics

Input impedance ($Z_{in}$) is what the signal source "sees" looking into the amplifier. You want this to be high so the amplifier doesn't load down the source and weaken the signal.

Output impedance ($Z_{out}$) is what the load "sees" looking back into the amplifier. You want this to be low so the amplifier can deliver signal to the load without significant voltage drop across its own output resistance.

Quick comparison:

The common collector's combination of high $Z_{in}$ and low $Z_{out}$ is exactly why it's the go-to buffer stage.

Frequency Response and Bandwidth

BJT amplifiers don't work equally well at all frequencies. The usable range is called the bandwidth.

• Lower cutoff frequency ($f_L$): Set by coupling capacitors and bypass capacitors in the circuit. Below this frequency, these capacitors stop passing signal effectively.
• Upper cutoff frequency ($f_H$): Limited by the BJT's internal junction capacitances ($C_{bc}$, $C_{be}$) and parasitic capacitances in the circuit. Above this frequency, these capacitances shunt signal to ground.
• Bandwidth: $BW = f_H - f_L$

A wider bandwidth means the amplifier can handle signals with a broader range of frequency content. Video amplifiers, for example, need wide bandwidth; a narrow-band RF amplifier does not.

Other Configurations

Darlington Pair

A Darlington pair cascades two BJTs so that the emitter of the first drives the base of the second. The result is a "super transistor" with:

• Extremely high current gain: $\beta_{total} = \beta_1 \times \beta_2$ (easily in the thousands)
• High input impedance and low output impedance, similar to a single emitter follower
• Common in high-current applications like power amplifiers and motor drivers

The downside is slower switching speed and reduced high-frequency performance, because the input capacitance roughly doubles and the first transistor's stored charge must be cleared before the pair can turn off.

Cascode Configuration

The cascode stacks a common emitter stage followed by a common base stage. This combination gives you:

• High voltage gain (comparable to or better than CE alone)
• Much better high-frequency performance than a standalone CE stage

The key advantage is that the CB stage on top shields the CE stage's collector from the output, dramatically reducing the Miller effect (the multiplication of $C_{bc}$ by the voltage gain). Less Miller capacitance means the amplifier's bandwidth extends to higher frequencies. Cascode designs are standard in RF and microwave amplifier circuits where both gain and bandwidth matter.