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How is an op-amp used in a sink circuit?
For a current sink circuit, the op-amp connection is changed, that is the negative input is connected to a shunt resistor. This will provide the necessary negative feedback to the op-amp. Then we have a PNP transistor, that is connected across the Op-amp output so that the op-amp output pin can drive the PNP transistor.
How does a voltage controlled current sink circuit work?
Voltage-controlled constant current sink circuit works exactly in the same way as voltage-controlled current source circuit that we built earlier. For a current sink circuit, the op-amp connection is changed, that is the negative input is connected to a shunt resistor.
How does the PNP transistor control the op-amp?
The PNP transistor will sink this 1A of current and the op-amp will use this voltage drop and get the desired 1V feedback. This way, changing the input voltage will control the Base as well as the current through the shunt resistor. Now, let’s introduce the load that has to be controlled into our circuit.
What happens to the op amp when the circuit is off?
When the circuit is off, it is fully discharged, and when it turns on the output will be VC and the current will be either off or lower than the target. The negative terminal of the op amp will be driven up with the op amp output.
How does the 741 op-amp output and sink?
Internals of the ancient 741 opamp. Source: Wikipedia. From the internal schematic of the 741 op-amp it should be clear that the output can source current from the \\$ V_{S+} \\$rail via Q14 or sink current to the \\$ V_{S-} \\$rail via Q20. In practice this would work as follows:
How does the op-amp turn on the transistor?
The output of the op-amp will turn on the transistor in a way that the other input will get 1V from our Vsupply. The shunt resistor will produce a drop voltage as per Ohms law, V= IR. Therefore, 1A of current flow through the transistor will create a drop voltage of 1V.
What does sinking current mean in an amplifier?
Sinking current means that current is flowing in to the op-amp from the load. Real op-amps have all sorts of non-ideal behavior; they have non-zero output impedance, they have distortion, and they cannot drive the output to the supply rails (even “rail to rail” outputs can’t do the last few tens of millivolts).