The LM324 is a cost-effective choice for an op amp, especially when you need to apply ground-level inputs. Although its output purportedly includes ground, its poor current-sinking capability limits the applications. At output voltages lower than 0.5V, the op amp's sinking current ranges only from 2 to 100 µA. You can add an external current-sinking circuit to bring the usable output voltage down to the millivolt level. In Figure 1, Q₁, Q₂, and R₃ form a 4-mA current source that drains the output of the LM324. R₄ is the load, demanding a sink current of 4 mA. This design uses a 2N2222 transistor for its low saturation voltage. The output characteristic becomes the saturation characteristic of the added transistors, Q₁ and Q₂. Using this current source, the output voltage is linear down to 22 mV above ground. Figure 2 and Figure 3 show the output characteristics. The lowest usable output voltage depends on the load (sink) current. When the load current is 0.5 mA (R₄=30 kΩ), the output voltage is linear down to 4 mV. Figure 4 is the original output characteristic of the LM324 driving R₄ (3.9 kΩ) without the added sinking current source. The current source presents a constant load to the LM324. You can configure a leftover op amp as a voltage comparator to cut off the current source when the output voltage is higher than 1V.

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