The circuit below shows an active low pass filter that is universally applicable. Only 4 equal (!) resistors needs to be changed to change the filter corner frequency fc. It uses only standard parts, is suitable for a large supply voltage range and needs only a modest 2mA of supply current.
R2, R3 and R4 set the input bias voltage at half the supply voltage. R5-R8 and C3-C6 are the low pass filter network. C3, C4, C5 are bootstrapped to provide a sharper roll-off. Q1 and Q2 are a series-feedback pair and function as a ‘super’ emitter-follower. A standard emitter follower has a gain of ≈ 0.90 … 0.95x. This ‘super’ emitter follower has a gain of ≈ 0.99x. Just like an OpAmp with feedback, in this series feedback amplifier, the feedback increases the input impedance, lowers the output impedance, stabilizes gain and lowers distortion. Not bad for a single €0.10 transistor that is added!
You cannot get it more simple than this, a 1 transistor CW filter. This filter is build according to the (surprise !) Multiple Feedback BandPass Filter (MFBPF) topology. Below, you see the circuit, originally designed by KE3IJ (alas now SK).
So after studying some designs of multiple feedback band-pass filters for CW by other designers (see my previous post), I’ve decided to try my hand at designing and building one myself. Actually, I’ve made two different versions. The first one is designed by myself and is a two stage filter with an fc of 750 Hz. The second one is the filter designed by Tom Hall GM3HBT and published in Ham Radio Today 1987. I selected this filter because the gain ( ~1x), the center frequency ( ~750 Hz) and the Q / bandwidth ( ~5x / 100Hz).
Design
Stages
R1 [ kΩ ]
R2 [ kΩ ]
R3 [ kΩ ]
C1=C2 [ nF ]
Gain [ ]
fc [ Hz ]
Q [ ]
BW [ Hz ]
13. GM3HBT
2-3
390
12
820
2.2
1.1
740
4.20
176
PA3COR
2
110
2.1
220
10
1.0
747
5.17
145
Overview of MFBP CW filter found in radio literature – PA3COR.nl
Band-pass filters are filters that are designed to let only a certain set of frequencies pass through. Overtime a myriad of different topologies have been developed by some very creative designers. Passive vs active, RC-filters vs LC filters, combinations of low pass and high pass filters, with gain and without gain, etc. One of the topologies that are often used for CW filters in receivers is the multiple feedback band-pass filter :
Recent Comments