Copyright ©1996-2004 GlassWare        All rights Reserved.

Grounded-Cathode Totem-Pole Amplifier

In this circuit, both top and bottom output tubes function as grounded-cathode amplifiers. The top tube's signal reference is the output, not ground, while the bottom tube's reference is ground, not the output. Because the top tube's input signal is superimposed upon its cathode's movement, no degenerative feedback is developed; and the bottom tube's input signal is referenced, not to its plate, but to ground. For example, forcing the output to go one volt positive will produce only a small countervailing decrease in conduction from the top tube (its grid will see the same positive one-volt signal as its cathode does) and an equally small increase in conduction from the bottom tube (the bottom tube's grid is also blind to the perturbation at the output). The only opposition will come from both tube's plate resistances (rp), in parallel, divided into the positive voltage perturbation at the output.

Functioning as grounded-cathode amplifiers entails both an increased output impedance and a worse distortion figure compared to the cathode follower configuration; however, it does offer gain, which can better fuel a negative feedback loop by increasing the total gain. In other words, this topology in itself is not suitable for driving low impedance loads without a global negative feedback loop.

 

(A loudspeaker's impedance is usually so much lower than the output stage's output impedance that little if any voltage gain is realized. However, in a headphone amplifier, this topology could produce voltage gain if the headphone's impedance were high enough and the output tubes' rp low enough.)

 

Where does the gain come from when the top tube looks like it is functioning as a cathode follower, as the load attaches to the top tube's cathode?

 

The answer is the same place the gain came from when the load attached to the plate: the transconductance of the tube gives rise to a change in current flow which, when this flow also travels through the load, creates a voltage across the load's resistance or impedance. Thus, the rough equation for gain is gm against the load: gain = gmRL. The top tube sees the same magnitude of input signal from its input to its output as the bottom tube sees. The only difference is that the bottom tube's voltage reference is ground and the top tube's reference is the output of the amplifier. If the load is a dead short to ground, then both top and bottom tubes become ground referenced and the drive voltage relationships become obvious.

 

Admittedly, when the load impedance is some value greater than zero, the drive relations become more difficult to see. Still, as long as we remember that the reference has moved from ground to the output, we stand a better chance of mentally unraveling the top and bottom drive voltage relationships.

More Information On Schematics