The LM324 operational amplifier is a cost-effective option, especially when you need to apply a ground level input. It is said that the output of LM324 includes ground level, but its current absorption capacity is very poor, which limits its application. When the output voltage is lower than 0.5V, the absorption current range of this operational amplifier is only 2 ~ 100mA. You can use an external current absorbing circuit to reduce the available output voltage to millivolts.
In Figure 1, Q1, Q2 and R3 form a current source to deplete the output current of LM324. R4 is the load and needs 4mA absorption current. This design uses 2n2222 transistor because of its low saturation voltage. The output characteristic of this design is the saturation characteristic of the added transistors Q1 and Q2. With this current source, the output voltage is linear until it drops to 22 MV above the ground level. Figs. 2 and 3 show output characteristics. The lowest available output voltage depends on the load (absorbed) current. When the load current is 0.5mA (R4 = 30 K Ω), the output voltage is linear until it drops to 4MV. Fig. 4 shows the output characteristics of LM324 driving R4 (3.9 K Ω) without increasing the absorption current source. This current source is a constant load of LM324.
In Figure 1, an external current source can reduce the available output level of LM324 to millivolt level.
Fig. 2, the transfer function of the circuit shown in Fig. 1 is linear; Up to millivolt level.
In Fig. 3, under the load current of 0.5mA, the output voltage is linear until it drops to 4 MV.
Fig. 4 shows the transfer function of LM324 without increasing the current source.
You can configure an unused operational amplifier as a voltage comparator to cut off the current source when the output voltage is higher than 1V.
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