27-11-2016, 12:18 PM
The output from a valve is a current. The load resistance turns it into a voltage.
But a virtual earth has zero ohms input impedance. That's why you don't see a voltage on the inverting input of an op-amp wired that way. But while there is no voltage, there is current. The input resistor in an inverting op-amp amplifier simply converts the input voltage into a current.
In this case, the virtual earth is at C3. Obviously, as far as the signal is concerned it's in parallel with R4, but R4 is about DC conditions, not signal.
A lot of transistor amps work this way. It's also Tim de Paravicini's favourite topology for a phono stage, whether implemented with valves, transistors, op-amps, or a mix of them.
Another example of a virtual earth in a valve stage is attached:
But a virtual earth has zero ohms input impedance. That's why you don't see a voltage on the inverting input of an op-amp wired that way. But while there is no voltage, there is current. The input resistor in an inverting op-amp amplifier simply converts the input voltage into a current.
In this case, the virtual earth is at C3. Obviously, as far as the signal is concerned it's in parallel with R4, but R4 is about DC conditions, not signal.
A lot of transistor amps work this way. It's also Tim de Paravicini's favourite topology for a phono stage, whether implemented with valves, transistors, op-amps, or a mix of them.
Another example of a virtual earth in a valve stage is attached:
