Transconductance Amplifiers


A transconductance amplifier takes an input in voltage and converts it to an output in current.  For a fixed resistive load the result is identical to a typical amplifier (i.e. a voltage amplifier), however for a varying or a reactive load the results are much different.  The relationship between the two is given by Ohm’s law V=I·R.

Given the wildly varying impedance curves of most loudspeakers, why would anybody want to use a transconductance amplifier?  The reason is that research shows a 20-30dB reduction in mid-band distortion is possible with current-driven loudspeakers.  This mode of operation is suitable for MF/HF drivers, but not for LF drivers as the current technology requires the majority of damping about resonance to be provided by the power amplifier.  It is possible this may change in the future if the benefits of current-drive are realized commercially (e.g. with the addition of a shorting ring to control resonance).  Please refer to this paper by Mills and Hawksford.

Distortion mechanisms reduced by current-drive:

• Thermally induced compression
• Nonlinear voice coil inductance
• Hysteresis from metal core
• Eddy current distortion

Another nice feature of transconductance amplifiers is built in overcurrent protection.  Since output current is the controlled variable, shorting the output will not increase the current level.  Also, an open circuit will simply result in amplifier clipping.  The reason for this is that, much as the ideal output impedance of a voltage amplifier is zero, the ideal output impedance of a transconductance amplifier is infinity.

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