I need Bandpass active filter and BSF as well using BJT (without opamp).

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iniarifin

Joined Dec 28, 2021
1
I wanna create one, and share this on my school forum. The thing is, i cant find any circuit with explanation about its cut-off freq.
 

crutschow

Joined Mar 14, 2008
29,466
What's BSF?

You could built a Sallen-Key active filter using a BJT emitter-follower in place of the usual unity-gain op amp, (with perhaps some small reduction in its performance).
You can cascade a low-pass filter followed by a high-pass filter to get the bandpass.
Google Sallen-Key filter.

(A nit, but I find it difficult to take a technical post seriously when it contains the word "wanna". :rolleyes: )
 
Last edited:

Ramussons

Joined May 3, 2013
1,227
What's BSF?

You could built a Sallen-Key active filter using a BJT emitter-follower in place of the usual unity-gain op amp, (with perhaps some small reduction in its performance).
You can cascade a low-pass filter followed by a high-pass filter to get the bandpass.
Google Sallen-Key filter.

(A nit, but I find it difficult to take a technical post seriously when it contains the word "wanna". :rolleyes: )
I think he means a Band Stop Filter, a Band Reject Filter.
 

Ian0

Joined Aug 7, 2020
4,822
The original Sallen & Key filter did use a single discrete device, the unity gain op-amp was a later adaption.
It was originally a triode valve connected as a cathode follower! Both bipolar transistors and FETs can be used successfully, although it does lead to an offset voltage of Vbe of Vgs.
 

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crutschow

Joined Mar 14, 2008
29,466
Both bipolar transistors and FETs can be used successfully, although it does lead to an offset voltage of Vbe of Vgs.
I have used an NPN emitter-follower driving a PNP emitter-follower to reduce the offset to a low value.
That, of course, doesn't work so well with complementary FETS due to their inherent differences in Vgs(ths).
 
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