Roger said:
AoE second edition, Fig15.3 p991
The circuit shows a differential amp used for thermocouples. It is a
standard diff amp arrangement with a T network added onto the front of
the feedback path.
The test says "It is just the standard differencing amp with the T
connection in the feedback path to get high voltage gain (200 in this
case) while keeping........:"
Why 200? Without the T network the gain would be 10. How do you get a
20X increase in gain by adding the T network?
What would the formula be for calculating the gain of such an amp? I
looked in the chapter where difference amps are covered, but there is
no mention of this circuit.
As others have pointed out, you need to do a proper nodal analysis of
the circuit to derive the formula for the gain. In its general form,
AoE's circuit looks like this (view entire message with a fixed-width font):
.. R2 R3
.. ___ ___
.. .---|___|---o---|___|---.
.. | | |
.. | | |
.. | .-. |
.. | | | R4 |
.. R1 | | | |
.. ___ | |\ '-' |
.. Va o-----|___|---o---|-\ | |
.. ___ | >----|-----------o-----o Vo
.. Vb o-----|___|---o---|+/ |
.. | |/ |
.. R1 | |
.. | |
.. | |
.. | R2 | R3
.. | ___ | ___
.. '---|___|---o---|___|---.
.. |
.. GND
..
.. (Created by AACircuit 1.28.6 Beta - 04/19/05 -
www.tech-chat.de)
R4 is the bridging resistor. For this circuit, the formula for the gain
is given below (I hope I haven't made a mistake...).
.. Vo R2 + R3 R2 R3
.. Av = --------- = --------- + 2 -------
.. Vb - Va R1 R1 R4
You can get this by solving the resistor network for the op-amp's (+)
and (-) inputs, with the usual condition that they should settle at the
same voltage (classical feedback control theory). See Win's post for a
quick'n'smart way of computing the gain.
Note the added term 2 * (R2 * R3) / (R1 * R4). Setting R4 to infinity
(i.e., removing it from the circuit) yields the usual diff. amplifier
formula Av = (R2 + R3) / R1.
Substituting R1 = 25k, R2 = 250k, R3 = 10k, and R4 = 1k we get
.. 250k + 10k 250k 10k
.. Av = ------------ + 2 ---------- = 210.4
.. 25k 25k 1k
--
Regards,
Costas
_________________________________________________
Costas Vlachos Email:
[email protected]
SPAM-TRAPPED: Please remove "-X-" before replying