Ensure A Fixed Bias Current For Gain Blocks

Maxim MAX4331

Ken Yang

Electronic Design

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 Mini-Circuits 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 two-channel system that requires matched gains. To combat this problem, the circuit of the Figure 1 produces a constant and uniform source of current that's independent of part-to-part variations.

Ensure A Fixed Bias Current For Gain Blocks
Figure 1. This simple circuit provides a supply current
that is uniform and unaffected by part-to-part
variations in the GALI-21 gain block.

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 = 2 Ω (a low resistance), R1 = 1 kΩ, and R3 = 62 kΩ. CN is an optional noise-reduction capacitor.

Materials on the topic


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