# How to build the voltage controlled current source based on modified Hawland current pump with FET?

#### DX400

Oct 26, 2017
6
Hi,

As noted in the subject, I have a problem with building of the voltage controlled current source with obligatory grounded load. The input control voltage is in the range from 0 to 5 V, the output voltage is up to 20 V and output current on the load up to 150 mA. The load is some semiconductor device( or material), the (light-emitting)diode for example. Other very important requirements are that the load must be grounded, the control voltage on input and the voltages on the output should be measured with respect to the one common point(ground).
I have already used the modified Howland current pump. The scheme is quite good for output current of 25 mA, but the suitable heat sink was required to protect the OpAmp. Also I have used the unipolar voltage supply of the OpAmp to obtain the 20 V of output. The another benefit of this scheme is that the output current does not depend on the load resistance.

How to modify the Howland current pump with the grounded load for output current up to about 200 mA (150 required with about 10 -40 mA in reserve)?

I have found the example of such modification in the Ref.https://www.edn.com/design/analog/4...nimize-errors-in-grounded-load-current-source. The Howland current pump based, with n-channel FET switch.

I have tried this scheme with OpAmp LM324 and FET IRLB8743, using resistors R1=R2=R3=100 kOhm, R4=104 kOhm, and obtain the output current up to 200 mA by varying of R5 (pot of 5 kOhm) controlling resistor.

But as I found from the circuit calculations, presented in the Ref, the output current is dependent on the load resistance. Hence, this current source is quite bad.

It is possible to improve the scheme to resolve this problem and to obtain voltage controlled current source with grounded load for output current up to about 150-200 mA?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,769
There are several ways to boost the output drive of an opamp. See e.g. this compendium.

You could also use parallel opamps as shown here.

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