This is the original circuit from the legacy school (Ubung) manual. The main differences are the NTC (15k instead of 6,8k) and the use of Ge PNP transistors.
And guess what ! This circuit performs much better. The meter can be easily tuned to the center position and it is very sensitive. Putting your hand (no touching !) above the NTC makes the meter deflect. Blowing some wind across it (to cool down) makes the meter deflect to the other direction.
It’s a cloudy day today and sometimes the sun peeps through the clouds. This is even visible at the meter reading !
A short circuit description :
- The meter is set up in a Wheatstone bridge.
The Wheatstone bridge consists of the resistor network as shown in this diagram :
With the potentiometer, we can set the meter needle to the center position. Now we know that the left arm of the bridge (10 k resistor and C-E resistor of T2) is equal to the right arm (P1 and serial resistors R5, R6 and R7).
Once this bridge is unbalanced (C-E path or P1 setting), the meter will deflect either to the left or either to the right.
T1 and T2 form a DC amplifier. A very small change at the base of T1 will cause a very large change in the C-E path of T2.
The NTC (in series with R2) together wit R1 form a voltage divider. Since the junction of this voltage divider is coupled to the Base of T1, a slight voltage change will be amplified and will cause a change to the Wien bridge.
Here’s the circuit build with the legacy parts :