In a logarithmic amplifier the gain depends on the input voltage and input current. The circuit shown in figure bellow uses a transistor as a variable feedback device determining the voltage gain as function of the input voltage and input current. The next formulas can be applied in calculations involving this circuit.
Formula.1
Voltage gain:
Vout =Vin x lg(Iin/Iout)
Where:
Vout is the output voltage in volts (V)
Vin is the input voltage in volts (V)
Iin is the input current in amperes (A)
Iout is the output current in amperes (A)
Formula 2
Input current:
Iin = Vin / R
Where: Iin is the input current in amperes (A)
Vin is the input voltage in volts (V)
R is the resistance in ohm (Ω)
Note: the output voltage cannot exceeds the power supply voltage. For common operational amplifiers 15 V is a typical limit.
