Current source enables op amp's output to go to ground
Edited by Bill Travis
Frank Chan, Vancouver, BC, Canada -- EDN, August 5, 2004
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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Neither links to figures work nor the pdf version downloads uncorrupted. Please send the pdf version to my email address if you can. thank you
Ubair Hamid - 2010-13-6 12:28:51 PDT -
Current source enables
op amp’s output to go to ground
As precision is not required the current source can be further reduced to a single transistor with injected base current. Simply omit Q2 and increase R3 approximately by factor HFE(Q1)/2 to allow for tolerance and temperature change. Use low saturation transistor with (narrow) HFE range specification.
If designed for minimum current gain at lowest temperature it may not save significant current at average operating conditions but still saves one transistor.
Dirk Gaede - 2004-16-8 08:09:00 PDT
