Logic gates can be used to solve logic problems. But with one extra feature feedback- they can do much more. The word ‘feedback’ is a good one to describe this type of connection. The output of a circuit is fed back to the input. Fig. 45.26 shows one example of this with two single transistor NOT gates or inverters. The output of gate I feeds into gate 2, and the output of gate 2 is fightback into gate I. On the circuit diagram, two voltmeters are connected to read the output voltages of the two gates. To see how this type of circuit works, consider what happens if the transistor in
gate I is off. Its input voltage is low, so there is little base or collector current. Its output voltage will be high, since it is a NOT gate. The high voltage is fed into gate 2. This high voltage provides a high base current, and thus an amplified collector current for gate 2. Transistor 2 is on.
The low output voltage of transistor T 2 is fed back into gate I, which is just hat is needed to keep transistor T 1 off. This is a stable state-it will stay like this.
But now think what happens if the input to gate I is deliberately made high by
linking it briefly with the high voltage line. The base current into transistor T 1 will be amplified, and it will turn on. The gate will have a low output voltage. So the input current to transistor T2 will be low, as will its collector current. Since gate 2 is a NOT gate too, its output voltage will be high. This is linked back to the input of gate I. So the link from transistor T 1 to the high voltage line can be disconnected (Fig. 45.27). This input is already being kept high by the output of gate 2. The second state, with transistor T 1 on and transistor T 2 off is also stable.
This type of circuit, with two linked transistors, is called a multivibrator. This may seem an odd name, but a later example will show that a circuit of this type can keep on changing from one state to the other and back again. This circuit does not keep changing. It has two stable states and is called a bistable multivibrator. This is usually shortened to bistable. A bistable is a simple memory circuit. It ‘remembers’ the last change imposed on the circuit. Fig. 45.28 shows a way of setting this up with two microchip NOT gates or
inverters. They use LEDs (light-emitting diodes) to show whether the gate output is high (LED on) or low (LED off). As the flying lead is moved from one input to the other, the LEDs change and stay changed when the lead is disconnected.