Basic grid circuit

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The grid base circuit (ger .: grounded grid ) is often used in high frequency applications basic circuit of a electron tube . The control grid of the tube is connected to ground and the input signal is connected to the cathode . This results in a relatively low input impedance . The analog basic circuit with field effect transistors is referred to as a gate circuit ; the corresponding basic circuit with bipolar transistors is called the basic circuit .

The grid base circuit is mainly used in power output stages of high-frequency amplifiers in transmitters , previously also in UHF oscillators and pre-stages. Usually, special triodes or disc triodes are used for this , which have a grid connection with low line inductance. In the case of disc triodes, the grid with its ring-shaped connection contact completely separates the chambers of the anode and cathode circuit from one another.

Color picture tubes are also controlled on the (three) cathodes; the purpose here is a more favorable mechanical structure in the tube neck with only one common grid connection . The control circuit must be able to supply the entire anode current. The RGB signals come from the matrix circuit . In the case of monochrome picture tubes, on the other hand, there is cathode or grid control.

When used as power output stages, more control power must be applied at low frequency than with the alternative cathode base circuit, the gain is lower. At high frequency, however, the situation is partially reversed; this is where the grid base stages have the following advantages:

  • no reaction from the anode circuit to the input circuit, therefore
  • no risk of feedback due to the phase shift between input and output signal. The amplifier can no longer act as an undesired oscillator according to the Huth-Kühn circuit .

The capacitive reaction on the grid voltage, which in the cathode base circuit can lead to a drop in the input impedance at high frequencies and even to positive feedback at even higher frequencies, is eliminated.

Since the feeding signal sources already have low impedances at high frequency (e.g. 50 ohm wave impedance ), the circuit complexity for impedance matching in the cathode circuit of the tube remains low.

One advantage of the grid-based circuit is the relatively low tendency to oscillate (self-excitation); the amplifiers work more stably than in the cathode-based circuit. This is particularly useful at very high frequencies in the VHF or UHF range.