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Designing a Cascode with Two 6922

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Designing a Cascode with Two 6922

Postby Augustica.com » 02 Aug 2013, 11:36

A cascode built on two triodes has a considerable similarity with pentode but has none of its disadvantages. A cascode circuit is shown below and its similarity to a pentode should be obvious. The upper grid looks like a screen grid and does indeed have3 a similar (though not identical) effect on circuits behavior. The lower triode 6922 operates into the load presented by the cathode impedance of the upper triode 6922 which is small. This causes its load line to be nearly vertical, resulting in very low voltage gain from the bottom half of the circuit and consequently very low input capacitance. This is the reason why the cascode was often used in high frequency circuits. Since lower 6922 triode operates like an ordinary gain stage, albeit with very low voltage gain, it is inverting. And since upper 6922 triode receives its input at the cathode it is non-inverting. Overall then, the circuit is an inverting gain stage, the same as a pentode.

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Cascode operates as follows - the upper tube has anode load resistor R4, however, instead of modulating signal by changing the grid voltage, and holding the cathode voltage constant, we vary cathode voltage and hold the grid voltage constant. The grid of the upper tube is biased to the voltage that is required for linear operation of the upper tube, and is held to AC ground by the capacitor C3.

The lower tube operates as a normal common cathode circuit, except that it has at its anode load the cathode of the upper tube. Because the lower tube has a low value of load resistance, it would generate considerable distortion if it were allowed to swing over a wide range. Conveniently, most of the gain is provided by the upper tube, and distortion of the lower stage should not be a significant problem.

It is appropriate to operate lower tube's anode at 75 Volt. Thus, if the high tention voltage is 285V, the voltage across the tube is 210V as shown on the 6922's diagram below. We can chose an anode load for the upper tube and draw a load line in the usual way. In our example R4 is 100KOhm, and Vg is -2.5V, giving us Va 76.5V. The corresponding anode current is 1.35mA.

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If the anode of the lower tube is operated at 75V, and the upper tube has Vg of -2.5V, then the grid of the upper tube must have potential of 72.5V. Because the grid of the upper tube does not draw any current, its voltage determines the operating conditions of the upper tube.

Attempts to investigate the lower stage using anode characteristics of the 6922 is not very helpful. Instead, we will use curves demonstrating correlation between anode current and grid voltage.

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We know that Va of the lower tube is at 75V, so we can look along the curve for Va = 75V (blue curve on the graph) until we come to the point where Ia = 1.35mA (upper and lower anode currents are equal), this is the operating point of the lower tube, and it gives us corresponding Vg of about 2.5V. Plotting the point (red dot) with coordinates Va = 75V and Ia = 1.35Ma on the anode characteristics gives Vg of approximately 2.5V. Accordingly, using Ohm's law, we find that R3 is 1.8KOhm.
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