Electronic Design

Ensure A Fixed Bias Current For Gain Blocks

RF gain blocks (amplifiers) are popular because they offer wide bandwidth, low noise, and ease of use. They're designed to operate with a fixed value of supply current because variations in supply current cause variations in the gain, compression point, and other crucial specifications.

Typically, a series resistor sets the supply current to a value based on the known value of dc voltage at the RF choke. That voltage, however, can vary from part to part. For instance, the GALI-21 amplifier (from Minicircuits Inc.) specifies a dc output ranging from 3 to 4.1 V.

Therefore, a fixed supply voltage, such as 5 V, allows the supply current to vary ±30%, causing a variation in gain. That behavior is particularly serious in a twochannel system that requires matched gains. To combat this problem, the circuit of the figure produces a constant and uniform source of current that's independent of part-to-part variations.

Active feedback forces the voltages at the op amp's inverting and non-inverting terminals to be equal. Thus, VR1 = VR2, and:

The current ratio is set by resistors R1 and R2, and the gain block's supply current is set by VCC, R1, and R3:

In the example circuit, the GALI-21 specifies a supply current of 40 mA. So with VCC = 5 V, choose R2 = 2Ω (a low resistance), R1 = 1 kΩ, and R3 = 62 kΩ. CNOISE E is an optional noise-reduction capacitor.

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