All About Circuits Forum Constant current source with transistor and op amp
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#1
04-22-2010, 09:41 AM
 nikomas Junior Member Join Date: Mar 2010 Posts: 11
Constant current source with transistor and op amp

hi everybody,

can someone explain to me how work constant current source with pnp transistor and op amp like is shown here

http://basicelectronic.blogspot.com/...e-circuit.html

#2
04-22-2010, 10:58 AM
 Markd77 Senior Member Join Date: Sep 2009 Location: Birmingham Posts: 2,750 Blog Entries: 1

The op amp is comparing the voltage across the resistor R to the voltage reference and keeping them the same by making the transistor conduct more or less.
The voltage across R is proportional to the current.
The unlabelled resistor is the load and it's resistance can be varied without the current through it changing.
#3
04-22-2010, 11:05 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,806

Firstly - there is a small error in the schematic. The feedback from the transistor emitter should go to the OP-AMP negative terminal and the reference input should go to the positive input terminal.

Does that correction provide any further help in understanding the circuit operation?
#4
04-22-2010, 07:38 PM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 21,994

Quote:
 Originally Posted by Alberto You will be better off using an adjustble voltage regulator (LM317 or LM338) wired as constant current supplier. ( see the regulator data sheet)
An LM317 might be easier if they require current in a range of 10mA to 1.5A.

However, if they need less than 10mA, they will either need to go to an LM317L which is rated for as low as 5mA, or use two regulators; one sourcing >= 10mA, and the other sinking >= 10mA. The current difference between the two can then be as low as zero.
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#5
04-22-2010, 08:39 PM
 nikomas Junior Member Join Date: Mar 2010 Posts: 11

Quote:
 Originally Posted by Markd77 The op amp is comparing the voltage across the resistor R to the voltage reference and keeping them the same by making the transistor conduct more or less. The voltage across R is proportional to the current. The unlabelled resistor is the load and it's resistance can be varied without the current through it changing.
can you explain with more details? why is constant current Ie=Vref/R? and why feedback from the transistor emitter should go to the OP-AMP negative terminal?
#6
04-22-2010, 09:52 PM
 bitrex Member Join Date: Dec 2009 Posts: 79

Quote:
 Originally Posted by nikomas can you explain with more details? why is constant current Ie=Vref/R? and why feedback from the transistor emitter should go to the OP-AMP negative terminal?
An op-amp always "fights" to keep its inputs at approximately the same voltage, when connected as a negative feedback amplifier - that is with the output connected in some fashion to the inverting or "-" input. Consider the equation for a differential amplifier, which an op-amp is:

$V_o = A(V_+ - V_-)$

Where V_o is the output voltage, A is the op amp open loop gain, V_+ is the noninverting input voltage, and V_- is the inverting voltage.

If you connect V_o directly to V_-, that is $V_o = V_-$, you get the following equations:

$V_- = A(V_+ - V_-)$

$V_-(1 + A) = A*V_+$

$V_- = \frac{V_+}{\frac{1}{A} +1}$

Most op-amps have extremely high open loop gain A at low frequencies...maybe 1 million. So the 1/A term becomes very close to 0, and you get approximately

$V_- = V_+$.

In the constant current source circuit, since the two input voltages to the op amp are trying to be equal, you can analyze it by assuming this condition is true. You then see that the voltage at I_e, which should be the negative input in the schematic must be the same as the reference voltage, and the op-amp output will supply an output voltage about 0.6 volts lower to make this true: the op amp automatically compensates for the voltage rise across the transistor. The emitter current will then be the voltage drop across the emitter resistor, which is Vref/R, and the collector will be the same times the factor alpha.
 The Following User Says Thank You to bitrex For This Useful Post: nikomas (04-23-2010)
#7
04-23-2010, 12:55 PM
 nikomas Junior Member Join Date: Mar 2010 Posts: 11

Quote:
 Originally Posted by Alberto You will be better off using an adjustble voltage regulator (LM317 or LM338) wired as constant current supplier. ( see the regulator data sheet) Alberto

I need to have constant current of 35mA through pt100 thin film resistor. That current will heat pt100 and it will have resistance of 135ohms. If i understand good i set that current with choosing resistor R=1.25V/35mA? what input voltage i need? Vin = 35mA*135ohm + 3V (Vdrop)?

Last edited by nikomas; 04-23-2010 at 01:24 PM.
#8
04-27-2010, 09:26 PM
 Darren Holdstock Senior Member Join Date: Feb 2009 Location: UK Posts: 262

It doesn't help in understanding this circuit in that it's all upside-down because of the ground-referenced load and PNP transistor. You'll need to think of Vcc as being your reference point, not ground. Just like in the old days when the only transistors available were germanium PNP devices, and cars had positive earths.

The voltage across the sense resistor R will be Vref. Simple as that. But here's something I found out the hard way - don't make the sense resistor too large, or the feedback will form a pole with the op-amps parasitic input capacitance, and you'll have gain peaking and/or instability. As a rough rule of thumb, below 100 ohms is good; lower than that is even better. If the sense resistor comes out in the calculations as larger than this then lower the input reference with a lower voltage Vref or a couple of resistors in a potdown arrangement.

The op-amp inputs appear backwards because the PNP signal inverts the signal to the feedback, so as it's already inverted this has to go to the non-inverting input.
#9
04-28-2010, 08:53 AM
 bitrex Member Join Date: Dec 2009 Posts: 79

Quote:
 The op-amp inputs appear backwards because the PNP signal inverts the signal to the feedback, so as it's already inverted this has to go to the non-inverting input.
I'm pretty sure it won't work the way it's set up in the schematic. The PNP isn't inverting anything - if the voltage from the op amp goes up or down the signal will appear in phase at the emitter (like an emitter follower) and appear in phase at the "+" input: positive feedback instead of negative. Imagine changing the transistor from PNP to NPN and swapping ground and the supply - still wouldn't work!
#10
04-28-2010, 07:21 PM
 Norfindel Senior Member Join Date: Mar 2008 Location: Buenos Aires, Argentina Posts: 235 Blog Entries: 1

mmm... Assuming it's at equilibrium, if Iout rises, more voltage drops in R, so the voltage at the emitter of the transistor (AMP+) lowers, lowering the output of the amplifier, which causes still more current to flow. Yeah, it seems like the amplifier inputs are reversed.

Correctly done, with negative feedback, the amplifier's inputs are going to be at the same voltage (this is the key to understanding opamp circuits with negative feedback). If AMP+ is at Vcc-Ref, then AMP- is going to be kept at the same level, which is the same that the bottom of R. The voltage drop over R is then going to be = to Vcc - (Vcc - Vref) = Vcc - Vcc + Vref = Vref. If the voltage drop in R is fixed, the current is fixed.
If for any reason the current raises (for example: transistor heating), the voltage drop in R will be more, AMP- will lower, AMPout will rise, and the transistor is going to conduct less current, regulating it

 Tags amp, constant, current, source, transistor

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