EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Clear as mud on November 13, 2013, 02:30:34 am
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I understand why a resistor is required to limit base current in a transistor switch or amplifier, but my question is what happens when the transistor is driven by an op-amp in a negative feedback configuration? Is it helpful to put a resistor between the op-amp output and the base of the transistor? Why?
I searched for an answer to this question, but didn't find anything. I found this page regarding constructing voltage regulators from op-amps: http://tangentsoft.net/elec/opamp-linreg.html (http://tangentsoft.net/elec/opamp-linreg.html) In the first schematic on that page, there is no resistor between the op-amp and the transistor, but the final three schematics all have 10 ohm or 47 ohm resistors. Each circuit on the page is supposed to provide improved performance (in terms of output stability and noise) than the preceding circuit, but there is no explanation of how the resistors at the base of the pass transistors help.
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The resistor is there to protect the op amp and limit the output current it can supply.
As an extreme example, if you think about what happens when you short the output, you have a base-emitter PN junction (ie a diode) going to ground. The output voltage will be lower than the set point & negative feedback will drive the opamp output to the positive rail (or as close as it'll go). Without a resistor there, you're essentially shorting the positive rail to ground through your opamp and a diode. If you're lucky the opamp's short circuit protection should kick in and save it, but it's not exactly the nicest thing to do to it.
Unfortunately you can't just stick a big value resistor there either. The current flowing through the emmitter is the sum of the current through the collector and the base. The transistor's base sees a current of Ib = 1/(1+Beta)*Ie (Approximately Ie/Beta). If you are trying to pull a 1A load through a power transistor with a Beta of say 50, you'll end up pulling ~20mA through the base. If you try and drive that with a 1k resistor on the base that's an additional 20V your opamp will have to source in addition to the (Vout + 0.6V) it has to supply to drive the transistor.
This and the fact that your op-amp has to supply a relatively big load to drive the transistor on its own is why you'll usually see Darlington output stages with much larger gains or alternatively a FET output.
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A transistor amplifier without a small resistor in the base is usually called an oscillator.