# Voltage feedback and current feedback.

1. Jun 5, 2009

### bitrex

Looking at the equations for the gain of a common cathode tube amplifier with the cathode resistor unbypassed (though the question in general could apply to similar configurations with BJTs and FETs) I see that the gain of the amplifier is something like $$\frac{\mu R_{l}}{R_{p} + R_{k}(\mu +1) + R_{l}}$$, where Rl is the load resistance, Rp is the dynamic plate resistance, mu is the amplification factor, and Rk is the unbypassed cathode resistance. This is degenerative feedback where the feedback is proportional to the current through the device - the gain of the amplifier is reduced and the dynamic plate resistance is increased. However, in a cathode follower, when the emitter resistor is unbypassed the dynamic plate resistance is decreased by the factor $$\frac{R_{p}}{\mu +1}}$$. This must be voltage feedback since the input impedance is increased and the output impedance is reduced. What I can't see is how the feedback changed from current feedback to voltage feedback by just taking the output from the cathode instead of the anode.

2. Jun 5, 2009

### cabraham

It's current feedback, cfb, because the error signal is a current. In a voltage feedback loop, vfb, the error signal is a voltage.

With vfb, the output voltage (or current with a sense resistor) is fed back through a resistive divider, to the base of the input transistor which is driven until the differential voltage across the 2 bases (input is a differential pair) is reduced to virtual zero.

With cfb, the output voltage (or current with a sense resistor) is fed back through a resistive divider, to the common emitter of a totem pole emitter follower. The upper emitter sources current and the lower emitter sinks current. The output drives the feedback current to virtual zero when the loop is closed.

It's all about the error signal. The terms cfb & vfb refer to the error signal being a current or a voltage resp. Does this help?

Claude