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.
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.