Hybrid Cascodes

Cascode topology, used in both solid-state and vacuum tube circuits, increases the gain and extends the bandwidth of an amplifier.

When used in a hybrid circuit, cascoding can increase the useable output voltage swing of a tube circuit. For example, let's say we want to replace the 7199 input tube and circuit on a Dynaco ST-70 with something better. The EL34 output tubes need about +/- 35 volts of drive voltage to be brought to full output. The input voltage is about 2 volts peak to peak. Thus, the first stage needs to provide a gain of at least 35, and if feedback is used, something like 250 would be better.

The MOSFET's high transconductance provides a low source impedance that locks the triode's plate at a fixed 100 volts, which serves to preserve much of the 6DJ8's transconductance. The MOSFET also provides for a large voltage swing, as it continues to conduct with only a few volts across its drain and source. A triode, on the other hand, needs much more cathode-to-plate voltage to continue to conduct. (The 22k resistor that attaches to the plate of the input tube serves to greatly linearize the cascode stage and to greatly increase the gain by allowing a large valued plate resistor, while the 6DJ8 runs under a high idle current in the linear portion of its operation.)

