Need help with voltage to current circuit!!!

[tempvipguest]

Hi，everyone,

I have a reference voltage to current circuit to drive VOA.As follows,

The resistor R1 can be removed,So what is the purpose of R1?
Thanks a lot!

 

[mrinalmani]

R1 provides thermal stability to the BJT. It helps to reduce the leakage current due to minority carriers at elevated temperatures.
Although it can be removed, its good to use it.

 

[crutschow]

If the circuit is always in the active region then R1 serves no purpose and can be removed. Any transistor base leakage current will be absorbed by the op amp (rail-rail) output.

 

[KlausST]

Hi,

to improve precision i recommend to use a mosfet instead of BJT,

With a MOSFET there is about no gate current. But with BJT you have an bas current.

The Feedback is with R49. R49 sees load current + base current. So base current causes error.
Additionally the R1 causes current through R49 giving additional errors.

****

To improve stability, i´d replace the R1 with a c (best a foil c). An additional resistor between this C and R49 may further improve stability.

The OPAMP needs to be unity gain stable.


Klaus

PS. look for 4..20mA constant current circuits.

 

[schmitt trigger]

I'm with Klaus on this one, a Mosfet will yield more accurate results.

Although the current is low, and therefore the Mosfet's channel does not require too much enhancement, I would still use a logic level Mosfet.

 

[BrunoARG]

I don't understand the R1 connection; does it determine the voltage to current ratio, or does it gives stability to the transistor? Wouldn't R1 voltage be aproximately 0.7V (VBE)?

I think that the transistor base current is controlled by the opamp output voltage, but I don't know how the emitter can be connected like that.

 

[tempvipguest]

thermal stability？It is still difficult for me(a beginner) to understand.Would you please explain it in detail.
Thanks!

R1 provides thermal stability to the BJT. It helps to reduce the leakage current due to minority carriers at elevated temperatures.
Although it can be removed, its good to use it.

 

[tempvipguest]

If the circuit is always in the active region then R1 serves no purpose and can be removed. Any transistor base leakage current will be absorbed by the op amp (rail-rail) output.

When R1 will work?And how it works?
Thanks.

 

[KlausST]

Hi,

Opamps usually have totem pole outputs, With that I don't see any benefit of R1.

Klaus

 

[tempvipguest]

Hi,

to improve precision i recommend to use a mosfet instead of BJT,

With a MOSFET there is about no gate current. But with BJT you have an bas current.

The Feedback is with R49. R49 sees load current + base current. So base current causes error.
Additionally the R1 causes current through R49 giving additional errors.

****

To improve stability, i´d replace the R1 with a c (best a foil c). An additional resistor between this C and R49 may further improve stability.

The OPAMP needs to be unity gain stable.


Klaus

PS. look for 4..20mA constant current circuits.

Yes,it is quite right to use a mosfet instead of BJT.But for the application,the current error is quite small and can be negligible.So i still use BJT.

- - - updated - - -

I'm with Klaus on this one, a Mosfet will yield more accurate results.

Although the current is low, and therefore the Mosfet's channel does not require too much enhancement, I would still use a logic level Mosfet.

Yes.I think about MOSFET too,and a current limit resistor is added before MOSFET.

 

[crutschow]

When R1 will work?And how it works?
Thanks.

The circuit will work equally well with or without R1.
As I stated it serves no purpose in that circuit.