Common Grid Triode Tube Amplifier

The common grid triode vacuum tube amplifier is is shown on Picture 1. The triode used in this circuit is the ECC83 tube, or 12AX7. The power supply for the circuit is Us = 250 V DC. The heater of the triode is not shown in the circuit, but is standard triode heater at 6.3 V AC or DC and one triode heater consumes 150 mA current. The power supply voltage is leaded to the anode of the triode through the anode resistor Ra = 100 kOhms. The cathode of the tube is connected to the ground through the resistor Rk = 1 Kohm, and here the electrolytic capacitor is removed in order to not affect the input signal which is leaded to the cathode. For proper polarization of the grid the resistors Rm = 1 MOhms and Rg = 10 kOhms takes place. The amplified output signal is taken from the anode of the tube through the coupling capacitor C2 and leaded to the load, resistor Rload = 5 kOhms. The coupling capacitors are both C1 = C2 = 1 uF.

Picture 1: Common Grid Triode Tube Amplifier Circuit


Time-domain analysis

The results of the transient analysis in time domain for this circuit are shown on Picture 2. The green color line plot is the output voltage wave form and the blue color line plot is the input voltage wave form. The input voltage source is with sinusoidal wave form with amplitude of 100 mV at frequency of 1 kHz. The output varies from about +/- 0.8 V amplitudes, it is in phase with the input, but delayed for about 0.5 ms. The voltage amplification of this circuit is about Av = 8.5, and the current amplification is about Ai = 0.6. The current amplification is below 1. These are the results from the simulation:

For Vi max = + 100 mV => Vo max = + 827.23 mV;
For Vi min = - 100 mV => Vo min = - 858.75 mV;

--> Av = 8.5 (approximate voltage amplification Av = Vo/Vi)

For Ic1 min = - 272 uA => IRl min = - 177 uA;
For IC1 max = + 266 uA => IRl max = + 165 uA;

--> Ai = 0.6 (approximate current amplification Ai = Io/Ii)

Picture 2: Transient analysis - input and output voltage wave forms (time-domain)


Frequency-domain analysis

The phase-frequency characteristics of this common cathode circuit were measured with AC analysis in LT spice. LT Spice computes the small signal AC behavior of the circuit linearized about its DC operating point. In this AC simulation were used these parameters:

Type of Sweep: Octave;
Number of points per octave: 1;
Start Frequency: 20 Hz;
Stop Frequency: 10 MHz;

Picture 3: AC Analysis - output voltage [dB] and its phase [degrees] (frequency-domain)


The maximum voltage magnitude is about -1 dB at frequency of about 2.1 kHz with phase of 1.1 degrees, as shown on Picture 3. At frequency of 466 Hz the magnitude decreases at -4 dB with phase of 44.9 degrees and group delay of 194 us. So, the low frequency limit for this circuit is fL = 466 Hz. The magnitude decreases for 3 dB again at frequency of 9.2 kHz with phase of -38.7 degrees and group delay of 8.8 us. This is the high frequency limit for this circuit fH = 9.2 kHz. At frequency of 20 Hz the magnitude of the output voltage is -28.2 dB with phase of 94.4 degrees and group delay of 949.1 us. At frequency of 10 MHz the magnitude of output voltage is -21.5 dB with phase of 126.2 degrees and group delay of 38.9 ps. According to the results of the AC analysis this circuit has relatively small frequency band from low to high limit, which is less than 9 kHz.