The bias current varies according to the resistor and threshold voltage tolerances. Additionally, thermal noise

associated with RS couples directly into the gain of the circuit, degrading the overall noise performance. To

improve the circuit in Figure 9-4, use a current-source biasing scheme. Current-source biasing removes the

JFET threshold variation from the biasing scheme, and allows for lower-value filtering capacitance (CS) for

equivalent filtering due to the high output impedance of current sources.

V

+

JFE150

V

IN

R

D

+

±

+

±

+

±

RG

VOUT

CS

CS

I

BIAS

V

±

Figure 9-4. Common-Source Amplifier With Current-Source Biasing

9.1.4 Composite Amplifiers

The JFE150 can be configured to provide a low-noise, high-input impedance front-end stage for a typical op

amp. Open-loop transistor gain stages shown previously suffer from wide gain variations that are dependent

on the forward transcondutance of the JFE150. When precision gain is required, the composite amplifier (JFET

front-end + operational amplifier) achieves excellent results by allowing for a fixed gain determined by external

resistors, and improving the noise and bandwidth of the operational amplifier. The JFE150 gain stage provides

a boost to the open-loop performance of the system, extending the bandwidth beyond what the operational

amplifier alone can provide, and gives a high-input impedance, ultra-low noise input stage to interface with high

source impedance microphones.

Figure 9-5 shows a generic schematic representation of a current-feedback composite amplifier. The component

requirements and tradeoffs are listed in Table 9-1.

VDD

JFE150

V

IN

RD

R

S

+

±

+

±

+

±

CS

CS

+

±

+

±

+

±

RS2

RS2

OPA

R

G

VOUT

C

F

C

F

CD

CD

VBIASO

C

B

C

B

R

B

Figure 9-5. Low Noise, High Input Impedance Composite Amplifier

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JFE150

SLPS732B – JUNE 2021 – REVISED APRIL 2023

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