Using Operational Transconductance (Voltage-to-Current) Amplifiers

so there is with OPA861 finally a much improved successor of the old unfortunately discontinued Intersil CA3080 or its also discontinued superior successor the CA3280; I liked the CA3080 very much, designing voltage conntrolled amplifiers, or in my case a voltage controlled integrator was a snap, so maybe I´ll use OPAs861 in my future design, there are however two drawbacks of the OPA861:

1) Transconductance nonlinearity versus input voltage - fig 3 in datasheet, which even increases as control current is reduced - fig 4 in datasheet, such the linear input voltage range is very limited (similar with old CA3080) - so in my view an other IC implementing a linearization technique like in the CA3280 would be a good idea

2) tuning range of transconductance vs. quiescient currentis very limited - to 1decade as I deduce from datasheet fig. 2, the CA3080 could be tuned linearly over 3 decades, so my voltage controlled integrator worked very well over three decades without any component switching - so in my view an IC with larger tuning range would be a good idea

Hi Andree,

Thank you for your feedback.  Here are some additional notes that may prove useful to you.

1) The OPA861 is using a very different architecture than the CA3080 or the CA3280 and as such teh lienarization technique used in the CA3x80 does not apply to the OPA861.  Let me elaborate a little.  The CA3x80 input is essentially a differential pair.  the OPA861 is using an open-loop buffer.  It is possible to increase the linearity of the OPA861 to any accuracy required, the trade-off by doing so is the reduction of the Gm and possibly an increase in noise.  To increase the linearity, all is required is to insert a resistance on the E-output.  Below is a table of the achieavable linearity of the OPA861 when increasing the E-input resistor.

2) If your application can accept some loss in bandwidth, you may be able to use the OPA861 over two decades as the OPA861 was not evaluated under those conditions.  I believe the spice model can provide you an accurate prediction the behavior of the device.

thank you Xavier for your hint on linearization and tunability of the OPA861

yes the E-input/output of the OPA861 is quite different to the Minus-Input of the CA3x80, first is low impedance / high current as the Minus-Input of a current feedback amplifier, while the later is high impedance / no current as the Minus-Input of an OPA, so the CA3x80 and OPA861 can be quite similarly used as OTA in a VCCS or CC2+ way, while the operation of OPA861 in Transconductor / Macro-Transistor / Diamond-Transistor - way has no equivalent on the CA3x80s

Ok, so if I need a high impedance Minus-Input in my OTA-Circuit I can use the OPA 860 and connect the Buffer in front of the E-Input/Output and some resistor for linearization in between of them