2N2222-PIC16F1716 Variable Gain Amplifier - A Quantitative Test

The relationship between control voltage and amplifier gain is an important performance measure of an variable gain amplifier (VGA). In this test, The amplifier is feed by the 8-bit DAC output of 16F1716 microcontroller, the micro is re-programmed each time to generate a different control voltage.

Input frequency=455KHz, Input power=-37dBm

The output level (RMS) is measured by oscilloscope and the relationship of control voltage and output level turns out like that.

On the other hand, let's compare to this, the datasheet figure of Analog Device AD603 Variable Gain Amplifier, the gain is very nicely linearly proportion to the control voltage. I don't know if this linearity is vital in radio IF amplifier application.

2N2222-PIC16F1716 Variable Gain Amplifier brief test

The latest version of 27MHz radio car remote control receiver. In any radio receiver, the IF gain stage must provide variable gain in order to couple with the dynamically changing channel loss. The modification from previous receiver version is minor, the only different is the addition of a buffer amplifier after DAC output, this buffer amplifier is needed because it was found that the DAC output could not provide sufficient current to drive the dual Variable Gain Amplifier (VGA) input. Just soldered the compoents for VGA IF stages, can't wait for doing some test before solder the antenna matching coil.

The latest version of 27MHz RC receiver (DUT)

The previous version of 27MHz RC receiver (VGA not working at all)

455KHz, -41dbm signal is feed into the VGA input port, the VGA can only provide a relatively small dynamic range, so it is easily overloaded.

The PIC16F1716 MCU is programmed to generate this linearly ramp up control signal. From 0V to 5V. DAC resolution is 8-bit.

The output of one of the differential output of VGA. Notice that the VGA is not conducting before control voltage is up to about 0.7V. after 0.7V the gain increase quite linearly as the control voltage increase. Result is quite as expected.

The next test should be the dynamic range. TBC...