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 pi R1 C2)
Where:
B = the forward current gain of the transistor
fo = the cut-off frequency of the high pass filter
pi = the greek letter pi and is equal to 3.14285
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
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