Page 205 - Complete Wireless Design

Amplifier Design

204 Chapter Three

3.8 VGA Amplifiers
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 oper-
ate properly until much higher frequencies are reached. However, as the fre-
quency 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 per-
formance, considering that a PIN has better intermodulation specs at higher bias
currents and, as high received signal levels result in increased attenuator IMD,
decreasing the bias current to attenuate the signal will add to this problem.

3.8.2 Design of VGA amplifiers
A VGA amplifier with low distortion for 10 MHz and above (Fig. 3.109). A low-cost
attenuator type of VGA that does not vary the bias of the amplifier is shown.
With an AGC voltage of zero the gain will be high for this stage; as the AGC
voltage increases, the gain will decrease because of the shunting effect of the
decreasing resistance of the PIN diode. However, the return loss will also
decrease, so an unconditionally stable transistor is the safest for this circuit.
Match and bias the transistor as normal, and calculate R as:
P
V 0.8
AGC(HIGH)
R
P 25 mA
where V maximum AGC voltage expected.
AGC(HIGH)
Another method is the best, and the most nonreflective, type of VGA:
Employ an absorptive attenuator design in front of any fixed gain discrete or
MMIC amplifier. An AGC voltage of zero at the attenuator’s DC control input
will result in low gain (even a negative gain), while a control voltage of greater
than zero results in a steadily increasing gain. The return loss will remain
quite usable up to very high attenuation levels. However, a rise in the noise
figure of a VGA circuit—whether bias or attenuator controlled—is unavoidable

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