Come, gather round friends and Op Amps, Cascade into the future

On the day we call Tuesday, quiet it was in the room
Kevin was tired, class would start quite soon
His birthday arrived without a great ruckus
Until Thor clanked his coffee gear in among us.

I made coffee for Kevin (and the class) on his birthday.  I might have put more than just coffee in.  Happy 21st Kev.


Today's lecture was a simple linear continuation on the Op Amps discussing we've been having for awhile.  We talked about cascaded Op Amps.  Cascaded Op amps are simply a group of Op amps that stack gains multiplicatively (a devious word to spell).  In contrast to that devious word, these Op amps are fairly easy.

A = A1*A2*A3

The cascade connection, it should be noted, can very easily saturate the Op Amp.  Don't do that unintentionally. We did a bit of practice with this.

LAB- Part 1

Temperature Measurement with a Wheatstone Bridge
In this lab, we designed an Op amp circuit with a wheatstone bridge and a difference amplifier.  Balancing the bridge was an integral part of this lab.  If the bridge doesn't get balanced, we almost get to voltages where the op amp is saturated. We designed a bridge circuit.

2. Wheatstone bridge and difference amplifier design

Since the thermistor was 10.7kOhms at room temp, we put an extra 700 ohms in series with the top left resistor to balance the circuit.  We chose resistors for the op amp circuit such that gain would be greater than 6, to amplify .4V difference to be amplified to 2.4V.  We therefore chose 6.8k resistors for R2 and R4, and 10K resistors for R1 and R3 for an associated gain of 6.8.

1. Data:



We achieved our target goals, with an input difference of .43V amplified to a difference of 2.2V.  We expected a gain of 2.9 Volts with the given resistance, which implies that either we failed to account for some resistance, or that our Op Amp was approaching saturation.  Since our feed on the op amp was 5V, we assume the former, that some resistance was unaccounted for.

Exercise 2- DAC

This lab was pretty neat, since we were introduced to transforming a digital signal to an analog one.  To do this, we use resistors that scale exponentially, and several input voltages.  We did an example problem. The digital signals are amplified such that each increment in the analog output corresponds to a one or a zero on the digital voltage inputs.  Notable: the "digital" voltage inputs must be a constant one or zero; if one input in half the expected input voltage, everything breaks.


We were also introduced to instrumentation amplifiers, which amplify the difference between two input signals.  These guys can come in single IC packages, but aren't cheap, about $2/ IC.
gain for these are:
Av = 1+2R/R_g