Hypothetical Variable Gain Amplifier

What I find is that in recent years, the term ‘Variable Gain Amplifier’ has changed in meaning, to correspond more to a ‘Variable Attenuation Stage’, after a fixed-gain amplifier. And this seems especially true, when applied to ‘IF Stages’ – ‘Intermediate Frequency Stages’ – Of a radio receiver. I’ve also observed that low-distortion technologies are preferred in recent years, as opposed to the high-distortion technologies that manufacturers were limited to, say, in the 1970s, when ‘AGC’ was first being marketed to consumers.

Yet, even with the technologies that are now available, there are sometimes added constraints. For example, if one wanted the variable-resistance component either to be optical – for lowest distortion – or, for that to be a JFET – easier to implement – then, this component might need to exist externally to an IC, just because the IC itself may be engineered only to allow for two complementary types of transistors, those being, an enhancement-mode N-channel MOSFET and an enhancement-mode P-channel MOSFET. Further, The properties of such MOSFETs can sometimes be inconvenient, in the form of high Threshold voltage, named ‘VT0′, which is the voltage required to make the transistors start to conduct. Practical values of VT0 may be more suited to logic circuits, than to the processing of low-amplitude, analog RF or IF frequencies. A thinner oxide layer for the entire IC can reduce the required VT0.

Yet, the possibility exists for even a MOSFET to operate in ‘Triode Mode’, which is a mode in which it is Not ‘Saturated’. This mode is achieved when:

VDS < VGS – VT0

The problem in trying to reach this mode seems to arise in the fact that if, VT0 is already a higher-than-desired voltage, VGS-VT0 is likely to be a lower-than-desired voltage-range, since VGS is also limited by the supply voltage.

In Triode Mode, a MOSFET effectively behaves like a variable resistor, which decreases in value as the Gate voltage continues to increase.

And so to summarize what form the task might take, to make the Variable Gain Amplifier monolithic with a MOSFET-based IC, I constructed the following, hypothetical diagram, which does not explicitly nail down what VT0 is supposed to be, nor the supply voltage:

Serge_VCR_3b.svg

 

What I seem to have noticed however, in order for the suggested IF stage to work, is that the actual signal should not have a ‘Peak Amplitude’ at the Gate of the last amplifier stage, greater than (0.1V). Yet, the feedback-loop itself, that adjusts attenuation, could play a role in keeping that peak amplitude close to (0.1V).

(Corrected 7/7/2019, 11h05 … )

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