using op-amp to provide active biasing for power transistor

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winglj

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active biasing

Hi, everyone. I have a question about what kind of op-amp I should use to design an active biasing for a power transistor.

Basically the I am going to employ a sensing resistor to capture the power supply current from Vcc to transistor. The voltage drop is then compared with a reference voltage by an op-amp. And the op-amp output is connected to the transistor base to provide negative feed-back.

I am wondering which op-amp I should use? Can the basic general purpose op-amp usable, since the bias circuit only require operating at DC. If so, which op-amp (band and part number) do you recommend?
 

opamp active bias

What I understand, you can use any general purpose Op-Amp like uA741, LM324 etc etc.

ENJOY!
 

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    winglj

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transistor baising in 741 opamp

An LM358 dual and LM324 quad opamps can measure a voltage that is at their negative supply voltage which can be ground.
A 741 opamp will need a dual-polarity supply to measure a voltage at ground.

Opamps have a fairly low output current which might not be enogh to drive your transistor.
 

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    winglj

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op amp transistor output

Audioguru said:
A 741 opamp will need a dual-polarity supply to measure a voltage at ground.
Click to expand...

Hey Audioguru - the 741 is not "lausy" anymore ? Congratulations !
 

using an transistor over an op amp

LvW said:
Hey Audioguru - the 741 is not "lausy" anymore ? Congratulations !
Click to expand...
It is not good for audio because its full output level bandwidth is to only 9kHz and it is noisy.
 

low output current op amp


I agrre with you, more or less. Don´t be confused - I was only joking.
Be happy , don´t worry.
 

54** opamp

Well, since my op-amp is only working for DC comparison, actually I think the bandwidth is not a big issue. The problem actually is to avoid any oscillations at high frequency.

One more question is that in the National Semiconductor website, for the LM324, it states that the input/output type is "Vcm to V-, Not R-R Out". What is that meaning? Does it mean that the output voltage swings from V- to Vcm? Thanks.
 

dc biasing using opamp

The old LM324 is not a modern Cmos opamp that has inputs and outputs that are "rail-to-rail".
Rail-to-rail is from the negative supply voltage to the positive supply voltage (when the load current is very small).

The LM324 has inputs and outputs that work fine at the negative supply voltage but the positive "common-mode" voltage limit is 1.5V less than the positive supply voltage. The inputs do not work properly if their voltage is higher and the output voltage will not go higher.
 

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    winglj

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transistor output op amp

Hi, Audioguru. Thanks for your expert review~ Just one more question, if the input voltage is higher than the V+ - 1.5V, what will happen?

Since in my application, I need to supply the Op-Amp with V+=6.5V, V-=-3V (a little bit rare), and require the input voltage at around 6V, and output voltage swing from -2.5V to 1V.

So the output voltage high end is not an issue for me, but the input voltage is higher than the common mode voltage requirement. Does it affect the input resistant, offset voltage or output voltage when the input voltage is so high?

Thanks.
 

biasing op amps

winglj said:
if the input voltage is higher than the V+ - 1.5V, what will happen?
Click to expand...
The opamp will not work properly. The output of some opamps will suddenly go the in wrong direction.

The supply voltage of +6.5V is too low for the opamp to work properly with an input voltage of +6.0V. Can't you reduce the input voltage?
 

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    winglj

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using 741 with gnd and +v

I just download the LM324 SPICE model, it seems from the simulation, it is OK for 6V input and 6.5V V+. But I think it is better to bench test before make a conclusion.

Since I want to power the transistor and op-amp both using 6.5V Vdd, I don't want to reduce the input voltage, since that will cause the power dissipation at the sensor resistor for the transistor is too high.

The other option is use separate power supply for op-amp and transistor, for my case, it will be 15V, but make routing a little bit complicated.
 

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