Transconductance Amplifiers

[Anonymous]

An often overlooked device, the transconductance amplifier is a very useful component. Some obvious uses for it in PI design are voltage controlled amplification, active filter tuning, and even sample and hold circuits. At about 85 cents a pop the CA3080A is well worth a look at.
The transconductance amplifier is like an op-amp except that its output signal is described by its transconductance. The transconductance or gM is equal to the change in output current divided by the change in input voltage. You are correct if you see a similarity between it and a vacuum tube.
I was surprised to find a brand new (May 2002) set of application notes from Intersil. You can download this excellent work at: http://www.intersil.com/data/an/an6/an6668/an6668.pdf
The CA3080A would seem to be a good choice for an SAT circuit. I am sure that there are many uses for it for our PI designs. There is also a CA3080E available. I believe that it is the same part.

 

[Anonymous]

Dave
It is not obvious to me how you would use one of these chips in place of R2 in the diagram below. R1 and R2 are the resistors that set the cutoff frequency.
Robert

 

[Anonymous]

Robert,
Transconductance amplifiers are easiest to use in filters where one end of the tuning resistors are going to either ground or virtual ground such as at the input to an op-amp. My favourite filter to use is the state variable. This filter can simultaneously provide a High Pass, Low Pass, and a Bandpass output. By summing the High Pass and the Low Pass, you can add a sharp notch filter output.
For tuning low pass and bandpass filters I have often used analog switches to chop a resistor on and off. To do this you need a high frequency clock and a pulse width modulator. If the analog switch is in series with the resistor then the effective value of the resistor is proportional to the duty cycle. A 10K resistor being switched with a 50% duty cycle provides a 20K resistor. A 10% duty cycle provides a 100K resistor. The integrators in the filter get rid of the high frequency noise.
As most analog switches come four per chip it is simple to have four variable resistors which track together. True High Pass filters cannot use this method as they will pass the clock signal.

 

[Anonymous]

My favorite OTA is the LM13700 which has two OTA sections plus two independent Darlington emitter followers.
I have used this chip as a tunable biquad bandpass filter, demodulator, modulator, preamplifier, integrator, sample-and-hold, low frequency RF oscillator using a simple LC tank without taps, comparator, high-speed full wave precision rectifier, gain control, triangle-to-sine converter, .....
musta left out something.
--Dave J.

 

[Anonymous]

Dave J,
Those idiots at National Semiconductor made the LM13700 obsolete!!!!! The tracking between the two amps was much better than using two CA3080's. The design of the current mirrors was also vastly better. National have been making a lot of their best chips obsolete lately. Dave E. * * *

 

[Anonymous]

Dave,
I am using NE5517 dual OTA. This is absolutely
same as LM13700 and isn't an obsolete part.
Tony

 

[Anonymous]

Thanks Tony for the info on the NE5517. I just checked on Nationals web site. The LM13600 is obsolete along with the LM13700A. The LM13700 is however still in full production. I looked pretty hard but I could not figure out the difference between the LM13700 and the LM13700A. The continuing availability of the LM13700 is really good news as it is a great chip.