FET Fundamentals Unit 5 – Dual Gate MOSFET
Exercise 2 – MOSFET Voltage Amplifier
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to determine the operating
characteristics of an N-channel MOSFET amplifier by using a typical test circuit. You will verify
your results with an oscilloscope.
DISCUSSION
- The circuit shows an N-channel enhancement / depletion mode MOSFET configured for
combination fixed bias. - A combination fixed bias is provided by the source resistor and the fixed bias adjust (RADJ).
- The fixed bias adjust (RADJ) varies the gate bias voltage, which establishes the value of the dc
drain current. - When a MOSFET amplifier is properly biased, the drain voltage is approximately half of
VDD. - An N-channel common source MOSFET amplifier amplifies an input sine wave with
minimum distortion. The output signal is 180° out of phase. - A disadvantage of source bias is that it reduces the circuit gain. This effect is reduced by
using a capacitor to short the source resistor. - A coupling capacitor prevents any external dc voltages from upsetting the combination bias.
- The bias divider resistors can be large because the gate terminal draws very little current.
- The MOSFET amplifier is a voltage-control device. Input signal variations in amplitude
create variations in drain current which develop a signal voltage across the load resistor. - The ac voltage gain (Av) is determined by the ratio of the output voltage (Vo) to the input
voltage (Vi). Av = Vo/Vi - In this circuit configuration the input signal is at gate 1, while a dc level is applied to gate 2
to control the output signal.