Circuit diagram of triode inverter

 This is an inverter circuit, also called a NOT gate circuit in terms of logic circuits. The RB1 (15K) resistor in the figure is the input resistor. Its lower end is the signal input terminal and should not be connected to ground. It should be connected to the input signal (see the figure below). The RB2 resistor is the bias resistor. When the input terminal voltage = 0, RB1 and The RB2 voltage divider makes the base voltage of the G2 tube negative, causing the emitter of the G2 tube to be reverse-biased, ensuring that the G2 tube is reliably cut off. When the input voltage is positive (eg = +6V), the divided voltage values ​​of RB1 and RB2 are higher than those of G2 The emitter junction of the tube is forward biased, and the emitter junction is turned on, causing the G2 tube to saturate and conduct. See below:

What is drawn in the dotted box in the figure is the output circuit of the previous stage, which is connected to the input resistor RB1 of the host circuit. When the G1 tube of the previous stage is turned on, its output voltage = 0, and RB1 and RB2 divide the voltage, causing the base of G2 tube to be reverse biased. , so that G2 is cut off and its output is high voltage. When the front-stage G1 tube is turned off, its output voltage is high voltage (+6V). The voltage is divided by Rco, RB1 and RB2, which forward-biases the base of the G2 tube, causing G2 to conduct, and the output = 0.