This is educational tutorial on how to use transistor as clamping diodes. In this example of using transistor as clamping diodes it is illustrated how an input signal having amplitude from -2.5V to 2.5V with center at 0V can be converted to an output signal having amplitude of approximately from 0V to 5V with center at 2.5V. That is we want a clamping circuit which does the following:
We can easily achieve that using diodes or Schottky diode. This solution with 1N4148 diode is presented below.
The input and output signal waveform from the above diode clamp circuit is shown below.
Since a transistor internal structure has two diodes back to back with one being the BE junction and the other being the CB junction we can use either of them as diode in circuits. First we will show how to use the BE junction diode and then we will illustrate the CB junction. The CB junction diode of a transistor has more zener diode voltage so this is more effective as diode in electronics circuit applications.
Transistor BE junction as diode
The following circuit shows how to use a transistor BE junction as diode for converting a rectangular or any type of waveform with -2.5V/2.5V amplitude range to approximately 0V/5V amplitude range.
With the input and output signal the following shows the graph of the input and output signal.
Transistor CB junction as diode
In the similar manner as above we can use a transistor CB junction as clamping diode to convert an input signal of -2.5V/+2.5V amplitude signal to an output signal with 0V/5V amplitude range. The following shows the circuit diagram and the input and output signal waveform.
The input signal can be of any type like a sine wave and the output is also of the same type as input as illustrated below.
Removing the little negative offset
As you can see in the above waveform graph both of the circuit implementation gives a negative off of around 600mV which is due to the BE junction diode voltage or the CB junction diode voltage.
Now if we were to remove this negative off-set we then could use the following circuit shown below.
With this circuit implementation the remaining negative offset is removed and we get a 0V/05V at the output as shown in the waveform graph below.
So here it was illustrated how one can use transistor as diode. Here clamping diode is used as an application example of transistor as diode.Other application example of transistor as diode is in making logic gates which is demonstrated in the tutorial How to make logic gates with diodes?.
