BJT and MOSFET Amplifiers

Amplifier

A circuit that increases the amplitude or power of an electrical signal.

Transconductance ($g_m$)

The ratio of the change in drain current to the change in gate-source voltage in a MOSFET, or collector current to base-emitter voltage in a BJT (related to $1/r_e$). It represents how effectively the input controls the output current.

Input Impedance ($Z_in$)

The impedance seen looking into the input terminals of the amplifier. A high $Z_in$ minimizes loading of the signal source.

Output Impedance ($Z_out$)

The impedance seen looking into the output terminals of the amplifier when the input signal is zero. A low $Z_out$ is desirable for efficient transfer of the amplified signal to a load.

Voltage Gain ($A_v$)

The ratio of the output voltage amplitude to the input voltage amplitude. It indicates how much the voltage signal is amplified.

Current Gain ($A_i$)

The ratio of the output current amplitude to the input current amplitude. It indicates how much the current signal is amplified.

Bandwidth

The range of frequencies over which an amplifier operates effectively, typically defined by the points where the gain drops by 3 dB from its mid-band value.

Voltage Gain (Common-Emitter/Source)

$r_e = \frac{V_T}{I_E}$ for BJT, $V_T$ is thermal voltage. $g_m$ is transconductance.

$A_v \≈ -\frac{R_C || R_L}{r_e}$ (BJT) or $A_v \≈ -g_m R_D || R_L$ (MOSFET)

Input Impedance (Common-Emitter/Source)

$η$ is current gain for BJT.

$Z_in \≈ \β r_e$ (BJT) or $Z_in \≈ \∞$ (MOSFET)

Output Impedance (Common-Emitter/Source)

Assuming collector/drain resistor is dominant.

$Z_out \≈ R_C$ (BJT) or $Z_out \≈ R_D$ (MOSFET)

Voltage Gain (Common-Collector/Drain)

Unity voltage gain.

$A_v \≈ 1$

Input Impedance (Common-Collector/Drain)

$R_E$ is emitter resistor for BJT.

$Z_in \≈ \β R_E$ (BJT) or $Z_in \≈ \∞$ (MOSFET)

Output Impedance (Common-Collector/Drain)

Low output impedance.

$Z_out \≈ r_e$ (BJT) or $Z_out \≈ \frac{1}{g_m}$ (MOSFET)

Voltage Gain (Common-Base/Gate)

High voltage gain, phase inversion absent.

$A_v \≈ \frac{R_C}{r_e}$ (BJT) or $A_v \≈ g_m R_D$ (MOSFET)

Input Impedance (Common-Base/Gate)

Low input impedance.

$Z_in \≈ r_e$ (BJT) or $Z_in \≈ \frac{1}{g_m}$ (MOSFET)

Output Impedance (Common-Base/Gate)

High output impedance.

$Z_out \≈ R_C$ (BJT) or $Z_out \≈ R_D$ (MOSFET)