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• Ideal follower circuits have a voltage gain of one.
• The power and current gain can be very large (small input / large output).
• The input impedance is close to infinity.
• The output impedance is close to zero.
• Followers can be made using an op amp, a MOSFET or a bipolar transistor.

Op Amp Voltage Follower

MOSFET Source Follower

MOSTETs don't start conducting until the gate voltage is above 2 volts (or more for some devices). This problem is overcome by biasing or using negative feednack.

This circuit removes the biasing problem by including the MOSFET in the negative feedback path.

This is not an ideal way to control a motor because of the waste heat produced by the MOSFET. Variable pulse width digital switching is a much more efficient method.

Bipolar Junction Transistor Emitter Follower

Transistors don't start conducting until the base voltage is above 0.7 volts. This problem is overcome by biasing or using negative feednack.

This circuit removes the biasing problem by including the transistor in the negative feedback path.

This is not an ideal way to control a motor because of the waste heat produced by the transistor. Variable pulse width digital switching is a much more efficient method.