Voltage follower capable of up to 150mA delivery - what do I need?

Thread Starter

peskywinnets

Joined Jan 19, 2009
47
I hope to order the parts & trial this soon...ron re your circuit in post #56 could you explain what is happening on the latter part of the circuit ie to the right side of R2 onwards.

I'm curious why you are feeding back to the opamp +ve pin etc.
 

Thread Starter

peskywinnets

Joined Jan 19, 2009
47
Thank you .....such an arrangement was my first thought ....but this supply quite low voltage in the first place (3 x 1.5V batteries) & the leds are being run at 3.3V ...I'm not sure what the voltage loss is across such an arrangement (0.7V?), but it's likely going to mean that the batteries can only fade away a small amount before that circuit won't be able to follow the input voltage any longer.
 

Audioguru

Joined Dec 20, 2007
11,248
The two series complimentary emitter-followers I showed have almost no voltage loss.
The output voltage is almost the same as the input voltage.
But the output of the opamp that drives them must be at least 0.7V higher than 3.3V.
Most opamps also have a voltage drop.
 

Ron H

Joined Apr 14, 2005
7,063
I hope to order the parts & trial this soon...ron re your circuit in post #56 could you explain what is happening on the latter part of the circuit ie to the right side of R2 onwards.

I'm curious why you are feeding back to the opamp +ve pin etc.
The MOSFET is connected as common source, so it inverts. It and the op amp form a voltage follower that can only source current, but can get to 3.3V, while supplying hundreds of milliamps (if the LED will allow it) if the input is 3.3V (after filtering). The feedback is connected to the +input because the MOSFET inverts.
 

Ron H

Joined Apr 14, 2005
7,063
The two series complimentary emitter-followers I showed have almost no voltage loss.
The output voltage is almost the same as the input voltage.
This is true when the input is not near the rails.
But the output of the opamp that drives them must be at least 0.7V higher than 3.3V.
Most opamps also have a voltage drop.
If the input goes to 0V, the output can go no lower than ≈+0.7V. If the input goes to +3.3V, the output will go to ≈+3.3V.
 

Thread Starter

peskywinnets

Joined Jan 19, 2009
47
That OPA376 ...what are the main characteristics I should look for if I have difficulty sourcing one (I always try & avoid any opamp starting "OPA"...as theyre normally 3 times the price of similar variants!)
 
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Ron H

Joined Apr 14, 2005
7,063
That OPA376 ...what are the main characteristics I should look for if I have difficulty sourcing one (I always try & avoid any opamp starting "OPA"...as theyre normally 3 times the price of similar variants!)
I chose the OPA376 for the voltage-controlled current source circuit. The critical parameters were low input offset voltage, and rail-to-rail I/O. For the voltage output circuit, you shouldn't need low offset, which should open up your options considerably. I can't guarantee that your eventual choice won't oscillate in the circuit I posted. If you pick another one, we can try to simulate it, but if it doesn't oscillate, that's still no guarantee.:rolleyes:
 
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