A direct-conversion radio receiver required an audio gain of 450 from a pentode vacuum tube. A pentode has a high transconductance – that is, the ratio of the change in plate current to the change of the control grid voltage that caused it. To get high gain, however, it needs a high load impedance. RF applications with pentodes often used LC-tuned circuits in their plate loads in which the impedance at resonance and, therefore, the gain is high. It is typically impossible to implement a high load impedance using an untuned circuit because of the DC requirements of the tube.
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| Figure 1. | A 6AU6 pentode vacuum tube needs a quiescent plate current of approximately 5 mA. |
For instance, a 6AU6 pentode vacuum tube needs a quiescent plate current of approximately 5 mA (Figure 1). If the quiescent DC plate voltage is to be 60 V, the load resistance must be no more than 12 kΩ. The 0.5-MΩ plate resistance of the tube and the 1-MΩ load of the next stage are negligible with respect to the 12-kΩ load. With a transconductance of 3900 microsiemens, those requirements demand an audio gain of 45. You can easily achieve this gain with a triode tube.
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| Figure 2. | To get a high load impedance with an untuned plate circuit, you can use a transistor current source for the tube. |
To get a high load impedance with an untuned plate circuit, you can use a transistor current source for the tube (Figure 2). The transistor has no gain but functions as an active load for the tube and supplies the 5-mA plate current. You adjust the 500 Ω potentiometer to obtain 60 V DC at the plate. The gain of the circuit is approximately 450. This gain implies a 150-kΩ load impedance that the transistor supplies in parallel with the plate resistance and the resistance of the next stage. Alternatively, you can use two triode tube circuits in series, each having a gain of 21.
