Lab 11 - GilesVolmir/Skylar-Scott--308L-Junior-Lab GitHub Wiki

Op Amps 4/11/12

The power supply I am using has nominal values of ground, +5, +12, and -12. I checked with a multimeter and all are within 0.2V of their respective nominal values. The pin configuration is in the data sheet for the op-amp package we are using, and it should be noted that V+,V- refer to the power pins (usually +/-12V) and +IN,-IN refer to the actual inputs.

The one to one amplifier, or voltage follower circuit, is useful since it takes a voltage, which may be from logic circuits or something of the sort, and keeps it from getting changed by current drawn. In other words, if you want to draw current from something that can't sustain its voltage when current is drawn from it, put a voltage follower in the middle, and you can draw current.

In the schematics for the voltage follower circuit in the data sheet and the lab manual, there is one noticeable difference, the data sheet inserts a 10kohm resistor in the feedback from the output to -IN. By the the first "Golden Rule" (that +IN=-IN) and since -IN is connected to OUT, and +IN to the input, the output voltage will equal the input voltage. Also by the second rule (no current is drawn by +IN/-IN) no current is drawn from the input, and there is no current loop from the output to -IN (also since there is no current drawn here, means there is no voltage drop across the resistor in the data sheet's voltage follower circuit, so it acts the same)

I wired up the circuit with reverse biased diodes on +12,-12 to ground so if they are reversed or something, it won't ruin the op amp. Other than that I followed the old lab manual for the circuit diagram. The oscilloscope confirms that the voltage follower has the same voltage in as out. The gain begins to drop below 1 at about 4kHz, as well as to become phase shifted.

4/13/12

Summing amplifier. The schematic in the instructions makes a summing amplifier, which i test to sum and then negate the voltage. By conservation of charge and golden rules (+IN = -IN) the current through both input resistors is equal to the current through the output resistor, I1+I2=I3 but since all the resistances are equal, V1+V2=V3. and since the other end of the output resistor is grounded, then the voltage out is equal V3. it is also negative since current has to be flowing to the output, so it must be negative. The maximum output is 11.3V, which is comparable to V-, which makes sense.

Differential amplifier I built the amplifier but i am not able to get a common mode on the lines so I can't calculate the CMRR. it works because the voltage across the resistor after V1 is V1-V2/2 then the feedback resistor drops by the same amount so V2/2-(V1-V2/2)=V2-V1 This indeed seems to be differential since the scope did not need to share any ground with anything.