Electronics

Transistor as a Switch

Here specifically we will focus on BJT. BJT has three operating regions as Cut off, Active and Saturation. To operate BJT as a Switch we need the bias BJT in Cut off and Saturation region. In cut off mode BJT is in OFF condition and in Saturation mode BJT is in fully on condition.

Below video shows the implementation of BJT as Switch in Proteus simulation software. You can see how the LED is turning ON/OFF i.e. BJT is switching ON and OFF.

Below are the circuits states for ON OFF condition

Op-amp as an Inverting Amplifier: 

        Below are the circuit diagram is of inverting amplifier with input voltage as 1V. This circuit is implemented in Proteus design suite. You  can see the output as -5V as the gain of the inverting amplifier is 

                                                            -(Rf/R1) ...and 

                                           so is here -(50k/10k) = -5 

                                                Input = 1 V hence 

                                               Output Vo = 1*(-5) = -5V

In below figure input is changed to 2V and hence output is -10V.

Op-amp as an Non-Inverting Amplifier: 

        

               The gain of non-inverting amplifier is (1 + Rf/R1) hence the gain for below circuit will be 

                                                           1 + 50k/10k = 6

and see the corresponding output voltages.

Basics of RC Circuit

A series RC circuit acts as a Low Pass Filter. The cut off frequency of this LPF is given by

We have implemented this circuit in Proteus and seen the response for Square wave input for different frequencies

As we can see from the circuit R = 1kohm and C = 1nF hence the cut off frequency is 159KHz approx.

Case 1: Input frequency is 10KHz and Vpp = 3V and we can see the output on Oscilloscope  the output is square wave with 3V peak to peak.

Case 2: Here in below figure the input frequency is changed to 90KHz, as this frequency is also below the cut off frequency hence the output is almost 3V and the shape of the waveform is changed. You can see it typical charging and discharging waveform of capacitor. 

Case 3: Here input frequency is 130KHz and this is also below the cut off frequency.

Case 4: As in below case you can see the input frequency is 200KHz and is above the cut off frequency here the output waveform has become almost triangular shaped and output is also decreased below 3V.

You can see in the below figures as we are increasing the frequency to 490KHz, 1MHz and then to 9MHz then here the output has become almost zero.

You can also check the output voltage by checking the output impedance i.e. reactance 

                                                                Xc = 1/2*pi*f*C

hence                                                      Xc = 17.69 Ohm 

and as R1 is 1KOhm hence the output is very very less.