Schmitt Trigger Circuits

Op Amps Previously in ET1 we investigated the Operational Amplifier. The Op Amp consisted of V in V out A feedback loop O V V in V out Feedback Loop A Non Inverting Op Amp It is possible to tell if an Op Amp is Inverting or Non-Inverting by the location of V in. If it went to the positive terminal of the Op Amp then the circuit was Non Inverting and vice versa.

Op amps as Comparators When using the Op Amp in this format we need to consider the value of the Supply Rails it is connected to. The values of the inputs at the + and - terminals then determine if V out would be positive or negative. If A is higher than B then V out = – 9V If B is higher than A then V out = + 9V -9 V V out -9 V + 9 V A B

Op Amps as Schmitt Triggers Op Amps can also be made to behave in a very similar way to Schmitt Invertors. These appeared in ET2 where they solved the problem of a NOT gate hunting at its switching threshold. voltage time If a voltage signal varies at the switching threshold of an Invertor it can cause it to burn out.

Op Amps as Schmitt Triggers When using an Op Amp as a Schmitt Trigger we now have to consider both the voltage values at the input terminals and the voltage values of the dual rail supply. A feedback loop is also used. Inverting Schmitt Trigger 0 V V out 0 V + V V in V non - inv V in V out Switching thresholds

Op Amps as Schmitt Triggers We cannot assume that we know which voltage rail the Op Amp will take for it’s starting voltage. We only know that when V in exceeds V non inv then the circuit will INVERT and the output will equal the other voltage rail instead. Thus, we can recognise an inverting Schmitt Trigger by the fact that V in is connected to the negative terminal.

Switching Thresholds The switching threshold of a Schmitt Trigger does not have to be the same as the voltage rails available. In the diagram below we can see that resistors have been added to create a Potential Divider. By selecting the correct values of these resistor we can change the Switching Threshold of the Op Amp (the value of V non – inv ). V non – inv = V out x R 2 V non – inv = V out x 50 V non – inv = 10 Inverting Schmitt Trigger 0 V V out 0 V + 14 V V in V non - inv Two resistors added to divide the voltage 10 k 50 k R 1 + R V

Non Inverting Schmitt Triggers In order to create a Non Inverting Schmitt Trigger you must connect a reference voltage to the input (V ref ). This reference voltage is constant and thus the circuit will switch when the positive terminal (V in ) equals the negative terminal. The resistors can be treated as a voltage divider again to make calculations easier. Non - Inverting Schmitt Trigger 0 V V out 0 V + V V in V ref VpVp