Question

How do you calculate voltage gain, current gain, and input/output impedance in amplifiers?

Answer 1

In amplifiers, voltage gain, current gain, and input/output impedance are important parameters that describe the amplifier's performance. The methods for calculating these parameters vary depending on the type of amplifier circuit being used. I'll cover the basics for common amplifier configurations:

Voltage Gain (Av):
Voltage gain is the ratio of output voltage to input voltage and is usually expressed in decibels (dB) or as a unitless ratio.

For common emitter (CE) bipolar junction transistor (BJT) amplifiers and field-effect transistor (FET) amplifiers, the voltage gain (Av) can be calculated using the following formula:

Av = ΔVout / ΔVin

Where:
ΔVout = Change in output voltage
ΔVin = Change in input voltage

For example, if the output voltage changes by 5 volts when the input voltage changes by 0.1 volts, the voltage gain would be:

Av = 5 V / 0.1 V = 50 (unitless) or 20 * log10(50) = 34 dB

Current Gain (Ai):
Current gain is the ratio of output current to input current and is also expressed in decibels (dB) or as a unitless ratio.

For common emitter (CE) BJT amplifiers and FET amplifiers, the current gain (Ai) can be calculated using the following formula:

Ai = ΔIout / ΔIin

Where:
ΔIout = Change in output current
ΔIin = Change in input current

Input Impedance (Zin):
Input impedance is the impedance seen by the input signal source when connected to the amplifier's input.

For CE BJT amplifiers and FET amplifiers, the input impedance can be calculated by:

Zin = Vin / Iin

Where:
Vin = Input voltage
Iin = Input current

Output Impedance (Zout):
Output impedance is the impedance seen by the load when connected to the amplifier's output.

For CE BJT amplifiers and FET amplifiers, the output impedance can be calculated by:

Zout = Vout / Iout

Where:
Vout = Output voltage
Iout = Output current

It's essential to note that the above calculations are based on linear small-signal models of amplifiers. In real-world scenarios, amplifiers might have different behaviors in large-signal conditions and might have frequency-dependent characteristics as well. For complex amplifier circuits, these parameters may be calculated using circuit analysis techniques like Kirchhoff's laws and transistor models. Additionally, for more advanced amplifiers (e.g., operational amplifiers), these parameters might be specified in the manufacturer's datasheet rather than being calculated from circuit parameters.