Transistor high pass filter

It is easy to implement a simple high pass filter circuit using just one transistor and a handful of other components.


Transistor Circuit Types Include:
Transistor circuit types     Common emitter     Emitter follower     Common base     Darlington pair     Sziklai pair     Current mirror     Long tailed pair     Constant current source     Capacitance multiplier     Two transistor amplifier     High pass filter    

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Although operational amplifiers are able to form the basis of an active high pass filter, a single transistor is also able to provide the same function with very acceptable performance.

Sometimes it is more convenient to use a single transistor than use an op amp. In circumstances like this, the simple design given below can provide an excellent solution to an active high pass filter.

One transistor active high pass filter circuit

The transistor high pass filter circuit given below provides a two pole filter with unity gain. Using just a single transistor, this filter is convenient to place in a larger circuit because it contains few components and does not occupy too much space.

The active high pass transistor circuit is quite straightforward, using just a total of four resistors, two capacitors and a single transistor. The operating conditions for the transistor are set up in the normal way. R2 and R3 are used to set up the bias point for the base of the transistor. The resistor Re is the emitter resistor and sets the current for the transistor.

The filter components are included in negative feedback from the output of the circuit to the input. The components that form the active filter network consist of C1, C2, R1 and the combination of R2 and R3 in parallel, assuming that he input resistance to the emitter follower circuit are very high and can be ignored.

One transistor active high pass filter circuit
One transistor high pass filter

The equations for calculating the values in the one transistor high pass filter are given below:

C1 = 2   C2

R1   =   R2   R3 R2 + R3

So that the loading on the filter components is minimal and the caculations are not offset by the loading effect of the transistor itself:

R e ( β + 1 )   > >   R2   R3 R2 + R3

f o = 2 4   π   R1   C2

Where:
  B = the forward current gain of the transistor
  fo = the cut-off frequency of the high pass filter
  π = the greek letter pi and is equal to 3.14159

The equations for determining the component values provide a Butterworth response, i.e. maximum flatness within the passband at the expense of achieving the ultimate roll off as quickly as possible. This has been chosen because this form of filter suits most applications and the mathematics works out easily

When designing the circuit, a little iteration may be required to optimise the value so that available components can be used and impedance values, etc can fall within acceptable limts.


The simple two pole active high pass filter circuit enables a simple circuit to be incorporated into areas where it may not be convenient to use another approach. The simple calculations and the few components sued make it ideal to use.

This one transistor high pass filter circuit can be used when there is a need for a circuit to eleminate low frequency hum, but retain the high frequency audio, etc . .



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