My First Success

Well perhaps the first one that basically works.

I decided to try and get a Twin T network to work as a band pass filter. Here I demodulated the signal with a diode envelope detector:

The above graphs are for a 200mv RF signal modulated with a 1k Hz audio tone.

The problems with this circuit are:

• Low Q - Bootstrap to the network.
• High current consumption (22 mA) - Change the transistor DC bias arrangement.
• The CE voltage is about 900 mV.

But it does work!

I chose a frequency of 720kHz as there is a strong MW/AM station in Perth.

Update

Added a DC/AC bias components and tuned the envelope detector.

This would work in practice (but not well):

Tried adding an emitter resistor so I can bootstrap the network but the circuit will not have it. I think I have to add an additional transistor stage for bootstrapping.

Now the above Twin-T looks great but the network is near its optimal frequency (i.e 723 kHz) and tweaked to be just below the oscillation threshold). Move off this frequency and the Q dies dramatically or the network goes into oscillation (i.e. Gain >0dB and phase angle 180 degrees):

This is why the Hall network is so attractive.

AlanX