Preamplifier

We chose a CMOS hex inverter type HEF4069UBP to build the preamp and Schmitt trigger.

Here you can see the DC characteristic for a single gate.  

We chose
Vsupply = 9.00V
for all our measurements, as we are going to use a 9V battery.


The gate provides a maximum open-loop gain of about 25 at the centre of its transition region where Vin = 4.3V
(NB not Vsupply / 2)
   
We made a single stage amplifier with
an input resistor of 100k and 
a feedback resistor of 1M.  

The circuit is a classical shunt voltage feedback amplifier. We expected a gain of ten.

 However because the open loop gain is not very big we need to use a more complicated expression for the closed loop gain Acl.
Acl = (Rf/Ri+Rf) * (Aol/(1+BAol)

whence Acl = (1/1.1)*(25/1+.091*25) 

Acl = 0.909*25/3.27 = 6.94

 where B = Ri/(Rf+Ri)

 

YES! Acl = 7