"MAiSAKit" the Measuring Amplifier in Servo Amplifier Kit
Anyone trying to measure an op amp open loop gain (AOL) parameter using a "Force Input Measure Output" technique must measure the output voltages one at a time using two input voltages with different magnitudes then plug-in all the data gathered in the general formula of AOL = ΔVO/ΔVIN, typically from 100,000 to 1,000,000 and an ideal value of infinity. But op amps in real world do have an offset voltage (VOS or VIO)*, an unwanted voltage source in the input terminals that will affect the device under test (DUT) output voltage. Like a general purpose µA741 with typical VOS of 1mV is more than enough to saturate the output at either VCC or VEE due to inherent very high gain thus this kind of measurement method is impractical & futile.
The solution is servo loop test method which does the opposite "Force Output Measure Input" with the aid of null amplifier, remember you cannot directly inject voltage in the op amp output because it is a low impedance source aka voltage source. It controls the output to swing to 0V then a potential at the input is develop equal to the DUT's VOS to satisfy AOL (VIN - VOS) = 0V. The DUT input is amplified by the noise gain in the range of 100 to 10k (DUT dependent) to facilitate measuring even op amp with very low VOS (10 - 100µV, ex. OP07) using a decent multimeter (connected to the isolation resistor of null amp output) with 100µV maximum resolution. Open loop gain is the ratio of change in output to change in input; swinging the output from 0V to specific values the null amp will react accordingly that is an amplified version of the DUT input hence open loop gain can be computed with known output & measuring the DUT input (null amp output/noise gain).
The Measuring Amplifier in Servo Amplifier Kit is an analog educational kit, it employs an LF411 op amp as the null (aka servo) amplifier to control the DUT output and with negative feedback produces a corresponding voltage at the DUT input. The DUT control voltage is derive from a REF102 10V voltage reference & 2x amplification is more than enough to bring the DUT (@±15V supply) into ±saturation thus the capability to verify output swing performance. The DUT supply was designed to provide a total of 50V or ±25V (limited by LM317 and LM337) taking into consideration older op amps specified @±15V supply and ±10V common mode test condition.
The MAiSAKit with a laboratory DC power supply (capable of ±30V) and decent multimeter allows you to measure most DC parameters specified on the data sheet like offset voltage, bias current, offset current, open-loop gain, CMRR (Common-Mode Rejection Ratio), PSRR (Power Supply Rejection Ratio), output voltage swing, short-circuit current & supply current. An excellent material on the basics of testing op amps, part 1 to 4 written by David Baum & Daryl Hiser can be found at www.edn.com
* Another definition of offset voltage is the input required to force the output to 0V.