Improved connection method of triode switch circuit circuit diagram

The low voltage level we set may not necessarily turn off the transistor switch, especially when the input level is close to 0.6 volts. To overcome this critical situation, corrective steps must be taken to ensure that the transistor can be cut off. Figure 6 shows two common improved circuits designed for this situation.

Figure 6 Two improved circuits to ensure the transistor switching action is correct

In the circuit of Figure 6(a), a diode is connected in series between the base and emitter, thus increasing the input voltage value that can conduct the base current by 0.6 volts, so even if the Vin value is close to the value due to malfunction of the signal source At 0.6 volts, the transistor will not be turned on, so the switch can still be in the off state.

The circuit in Figure 6(b) adds an auxiliary hold-off resistor R2. Appropriate R1, R2 and Vin value design can ensure that the switch is turned off at the critical input voltage. It can be seen from Figure 6(b) that before the base-emitter junction is not turned on (IB0), R1 and R2 form a series voltage divider circuit, so R1 must cross a fixed (varying with Vin) divided voltage, so the base The voltage must be lower than the Vin value, so even if Vin is close to the critical value (Vin = 0.6 volts), the base voltage will still be pulled down by the auxiliary-blocking resistor connected to the negative supply to below 0.6 volts. Due to the deliberate design of the values ​​of R1, R2 and VBB, as long as Vin is in a high value range, the base will still have enough voltage to turn on the transistor without being affected by the auxiliary-cutoff resistor.

>>>Acceleration capacitor

Figure 7 Circuit with acceleration capacitor added

Sometimes the load of the transistor switch is not directly added between the collector and the power supply, but is connected as shown in Figure 8. This connection method is very similar to the circuit of a small signal AC amplifier, except that an output coupling capacitor is missing. The action of this connection method is exactly opposite to that of the normal connection method. When the transistor is turned off, the load is energized, and when the transistor is turned on, the load is cut off. These two circuit forms are common, so there must be a clear Discrimination ability.

Figure 8 Improved connection method of connecting the load to the transistor switching circuit

>>>Totem switch

totem switch

One of the transistor switch applications >>> driving indicator light

Figure 10(a) uses a transistor switch to indicate the output state of a digital flip-flop. If the output of the flip-flop is at a high level (usually 5 volts), the transistor switch is turned on and the indicator light lights up. Therefore, the operator can know the current working status of the flip-flop by just looking at the indicator light. , without using an electric meter to detect.

Sometimes the current capacity of the output circuit of the signal source (such as a flip-flop) is too small to drive the transistor switch. At this time, in order to prevent the signal source from being overloaded and causing malfunction, the improved circuit shown in Figure 10(b) must be used. When the output is at a high level, the emitter follower transistor Q1 is first driven for current amplification, and then Q2 is turned on to drive the indicator light. Since the input impedance of the emitter follower stage is quite high, the flip-flop needs to provide A small amount of input current can provide satisfactory operation.

Digital Display The circuit in Figure 10(a) is often used in digital displays.