Common Cathode Triode Tube Amplifier

The common cathode triode vacuum tube amplifier is the most used circuit configuration in the audio pre-amplifier applications. One simple common cathode amplifier is shown on Picture 1. The triode used in this circuit is the well known 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 parallel combination of the cathode resistor Rk = 1 Kohm and the electrolytic capacitor C3 = 47 uF. The input voltage signal is leaded through the coupling capacitor C1 and the grid resistor Rg = 10 kOhms to the grid of the tube. For proper polarization of the grid the resistor Rm = 1 MOhms 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 Cathode 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 +/- 0.9 V amplitudes, it is inverted from its input. The voltage amplification of this circuit is about Av = - 9, and the current amplification is about Ai = - 1800. The current amplification is huge as expected, because the grid of the triode tube consumes very low current, as it is here, +/- 100 nA. These are the results from the simulation:

For Vi max = + 100 mV => Vo min = - 919.56 mV;
For Vi min = - 100 mV => Vo max = + 885.47 mV;

--> Av = - 9 (approximate voltage amplification Av = Vo/Vi)

For Ic1 min = - 100 nA => IRl max = + 177 uA;
For IC1 max = + 100 nA => IRl min = - 183 uA;

--> Ai = - 1800 (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)


From frequency at 100 Hz to about 100 kHz the voltage amplification is about -0.8 dB and phase around -180 degrees, as shown on Picture 3. At frequency of 20 Hz the magnitude of the output voltage is -2.12 dB with phase of -152 degrees and group delay of 2.18 ms. So, we can state that the low frequency limit of this circuit is 0 Hz. The voltage amplification decreases for 3 dB at frequency of about 842 kHz with phase of -224 degrees, magnitude of -3.8 dB and group delay of 101.16 ns. This frequency is actually the high limit frequency for this circuit, fH = 842 kHz. At frequency of 10 MHz the magnitude of output voltage is -22.6 dB with phase of -271 degrees and group delay of -101.27 ps. According to the results from the frequency-domain analysis this amplifier circuit has flat voltage amplification in the audio band of frequencies, from 20 Hz to 20 kHz. That makes this circuit suitable for audio applications.