Op-amp LM324 characteristics

The datasheet for the LM324 op-amp has many useful values. Among these is the common-mode voltage or VCM. VCM is a term used to describe when both inputs to the op-amp are at the same potential. For the LM324 the range VCM can be is from 0 to Vcc-2. If the VCM is outside of this range, the output will not be correct.

Another important value for the LM324 is the open-loop gain or Aol. This value is represented on this datasheet as the voltage gain and it changes with temperature, Vcc and the output resistance. Using charts from the datasheet, it shows that the Aol can be estimated at approximately 110dB or 300V/mV.

Lastly, the offset voltage is a good design parameter to know. For the LM324 the typical value is 2mV while the maximum is 9mV at extreme temperatures. For design, the 9mV value should be used to include worst case scenarios.

A Basic Inverting Amplifier

We created a basic inverting amplifier using the following schematic.

We set VCM (or Common Mode Voltage) on this circuit using the R2/R1 voltage divider. For this circuit the VCM is ideally 2.5V. Because we are using 2 different voltage sources, it took some adjustment to get the VCM to match the DC component of the input.

You can see an AC signal is injected into VCM by the op-amp. This signal is 180 deg out of phase from the input and of the same magnitude. The capacitors C1 and C2 help to shunt this signal to ground. While the values of C1 and C2 are not very important, they should be within a reasonable range for the frequency of the input voltage. If 1pF capacitors are used with a 5Hz signal, some attenuation may occur.

You can see that the ideal closed-loop gain is 1V/V in this design. our actual output is very close to that using at 100mV AC input. The output is between 2.6 and 2.4 V DC, this is called the output swing. The output swing is centered around 2.5V DC.

Effects of R1 and R2

While the exact values of R1 and R2 are not very important, they can have noticeable effects on the circuit. First, it is important for the values to be very close or VCM will be higher or lower than 2.5V. This will move the output center and reduce the maximum output swing. Secondly, the values of R1 and R2 must be large enough to keep the circuit power consumption low. However, if the values are too large, the input bias current will start to have a noticeable effect on the voltage divider increasing VCM.

5k ohm R1 and R2	5M ohm R1 and R2

You can see that increasing the the value of R1 and R2 moved VCM so high that the output is now clipped. However, this effect can be compensated for by reducing the value of R1, this can be difficult due to the variations in the bias current. The large values of R1 and R2 can also be compensated by increasing VCC, shown in the next picture.

Another parameter that is important to the circuit operation is the input offset current. This value is the variation between the input bias currents. This can have an effect on balanced circuits.

Measuring input offset voltage

We used the following circuit to measure the input offset voltage on all four op-amps on the LM324.

This circuit inputs the exact same voltage to the non-inverting input and the inverting amplifier of the op-amp. The inverting amplifier then amplifies the input offset voltage by a factor of 20. To measure very small offset voltages, a larger gain may be used to increase the amplification to 100x or even 1000x. This can be accomplished by increasing RF to 100k or 1M respectively.

op-amp 1	op-amp 2

op-amp 3	op-amp 4

We used the math function on the scope to measure the difference in the input and output signals. This value is divided by 20 to obtain the actual offset voltages of 2.23mV, 1.45mV, 1.66mV and 1.82mV respectively. These are all well under the max value specified in the datasheet of 7mV at room temp.

Other observations

Another test we did on the LM324 was to measure the frequency response of the circuit at a gain of 20x and frequencies of 5Hz, 1kHz, and 100kHz.

5Hz	1kHz	100kHz

The output magnitude decreases dramatically when the frequency is increased to 100kHz. This is expected due to the Gain Bandwidth Product (GBP) of the Op-amp.

We also tested the effects of increasing the supply voltage to the Op-amp to 20V. The only noticeable effect was the gain increased slightly.

Vcc= 5V	Vcc= 20V