Common Anode Triode Tube Amplifier

The common anode 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 directly to the anode of the triode. The cathode of the tube is connected to the ground through the resistor Rk = 4.7 Kohm, and here the electrolytic capacitor is removed in order to not affect the output signal which is taken from 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 cathode 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. The input signal is leaded to the grid of the tube through the C1 and Rg.

Picture 1: Common Anode 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 +/- 0.08 V amplitudes, it is in phase with the input. The voltage amplification of this circuit is about Av = 0.8, and the current amplification is about Ai = 158. Here, the voltage amplification is below 1. These are the results from the simulation:

For Vi max = + 100 mV => Vo max = + 78.84 mV;
For Vi min = - 100 mV => Vo min = - 79.16 mV;

--> Av = 0.8 (approximate voltage amplification Av = Vo/Vi)

For Ic1 min = - 99 nA => IRl min = - 15.8 uA;
For IC1 max = + 99 nA => IRl max = + 15.7 uA;

--> Ai = 158 (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)


This circuit has a flat amplification from frequency at about 500 Hz up to 1 MHz, as shown on Picture 3. The magnitude of the output voltage in this band is -22 dB. At frequency of 31 Hz the magnitude decreases for 3 dB at value of -25 dB with phase of 44.8 degrees and group delay of 2.6 us. So, the low frequency limit for this circuit is fL = 31 Hz. The magnitude decreases for 3 dB again at frequency of 8 MHz with phase of -44.2 degrees and group delay of 9.8 ns. This is the high frequency limit for this circuit fH = 8 MHz. At frequency of 20 Hz the magnitude of the output voltage is -26.8 dB with phase of 55.6 degrees and group delay of 3.27 us. At frequency of 10 MHz the magnitude of the output voltage is -26.2 dB with phase of -51.7 degrees and group delay of 6.8 ns. According to the results of the AC analysis this circuit has relatively flat voltage amplification for the audio signals. The voltage amplification is below 1, but it is near one, and similar like the common collector amplifier, this circuit acts like voltage follower, the output voltage simply follows the input voltage. From the other side, the circuit has a relatively big current amplification.