howland circuit frequency range.

[zoulzubazz]

hey guys,



The image shows the two howland current source topologies. number two gives me a satisfactory response by pushing a current of 300uA through a 10kohm load when the differential amplifier i am using (ina105 ic) is supplied powered by +/-7.5V. but the frequency response degrades after about (input signal frequency) 10kHZ and the output current nearly halves at 100kHz. why does this happen? is this to do with the bandwidth and slew rate of the differential amplifier? will the performance improve if the AMP03 from Analog devices is used because it has better slew rate and bandwidth? would you recommend topology 1 or 2 for pushing a 300uA current through 10kohm load between 0-100kHz input frequency?
the following links show the characteristics of the two ic's in question. Thanks very much.

**broken link removed**
**broken link removed**

 

[crutschow]

Yes it may be due to the op amp's frequency response and slew rate.

What are the external resistor values you are using?

 

[zoulzubazz]

Thanks a lot. The value of the external resistor used is 1kohm and has a tolerance of 0.01%.

 

[FvM]

AMP03 should raise the -3dB cut-off frequency to about 200 kHz. This means, there will be a still a certain (smaller) gain drop at 100 kHz. The problem is that circuit loop gain is reduced by a factor 1k/10k, so loop bandwidth is considerably smaller than amplifier GBW.

 

[crutschow]

Reducing the circuit gain by increasing the value of R will also increase the output bandwidth. But that will have no effect if the op amp slew rate is limiting the output.