# Transistor high pass filter

### - a simple one transistor circuit to provide an active high pass filter

It is sometimes convenient to design a simple active high pass filter using one transistor. The transistor 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.

Transistor active high pass filter circuit

C1     =     2 C2

R1     =     R2 x R3 / (R2 + R3)

This is for values where the effect of the emitter follower transistor itself within the high pass filter circuit can be ignored, i.e.:

Re (B+1)     >>     R2 x R3 / (R2 + R3)

fo     =     1.414 / (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

By Ian Poole

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