# 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

<< Previous   |   Next >>

### Share this page

Rudy Ramos | Mouser Electronics
GaN’s Ground-Floor Opportunity
The electronics industry has a major role to play in helping to save energy, by enabling better equipment and new ways of working and living that that are more efficient and environmentally friendly. Maintaining the pace of technological progress is key, but improvements become both smaller and harder to achieve as each technology matures. We can see this trend in the development of power semiconductors, as device designers seek more complex and expensive ways to reduce switching energy and RDS(ON) against silicon’s natural limitations.

Forthcoming Events

. . . . More Events

##### Channels
Radio-Electronics.com is operated and owned by Adrio Communications Ltd and edited by Ian Poole. All information is © Adrio Communications Ltd and may not be copied except for individual personal use. This includes copying material in whatever form into website pages. While every effort is made to ensure the accuracy of the information on Radio-Electronics.com, no liability is accepted for any consequences of using it. This site uses cookies. By using this site, these terms including the use of cookies are accepted. More explanation can be found in our Privacy Policy