3.8.1 Introduction

Variable gain amplifiers (VGAs) can be designed in one of two ways: either by varying the active device s bias voltage to its base, which controls its collector current, and thus the gain of the transistor (see Automatic gain control ), or by placing a voltage- or current-controlled variable attenuator at the input to a fixed-gain amplifier. Since the latter design usually results in a more linear amplifier response over gain, especially with large input signals, it is preferred over the variable-bias design in many applications. An added disadvantage to the variable-bias type is that any modification to the bias of a transistor will also alter its S parameters. This means that not only will the gain be varied, but so will the return loss of the amplifier which can prove catastrophic if the VGA is attached to a filter circuit (a filter s response is dependent on its source and load impedance).

In using PIN diode attenuators, a few cautions are in order. As with most devices using PINs, the minimum usable frequency is normally above 10 MHz. Some special PIN diodes may attain lower frequencies, while some will not operate properly until much higher frequencies are reached. However, as the frequency is decreased in any PIN, its IMD and insertion losses will increase. As well, many PIN attenuator designs used for AGCs should be tested for IMD performance, considering that a PIN has better intermodulation specs at higher bias currents and, as high...