EDN Access -- 8.4.94 Composite amplifier improves nois

-August 04, 1994

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Design Ideas:August 4, 1994

Composite amplifier improves noise

Rea Schmid,
Comlinear Corp, Fort Collins, CO

Amplifying low-signal levels for signal processing often stretches a single op amp's abilities. A composite design can improve the input noise for detecting extremely small signals and can enhance output-drive ability. Both the op amp's noise terms and the thermal noise of the components limit the minimum noise in an op-amp-based design. Furthermore, amplifier noise figures are not at their minimum for low-impedance sources such as 50 Ohm. Using a transformer is a common technique to achieve matched input impedance for maximum power transfer. Fig 1 shows one example, which follows the transformer with a composite amplifier to achieve a lower input noise figure.

Fig 1's composite circuit provides a method of setting the input impedance at the noninverting input by dividing the 8.75-k Ohm feedback resistor by the forward gain plus 1. The circuit further divides this input impedance by the square of the turns ratio to match the source impedance.

The 1.5-k Ohm resistor and 2.6-pF capacitor control the frequency response of the transformer without adding noise. For lower gain operation, the resistor/capacitor network in parallel with the 75 Ohm resistor provides compensation to stabilize IC1. IC2 inverts the signal and provides stability for the overall composite design. The 47-pF capacitor that splits the 250 and 83 Ohm gain-setting resistors adds a pole for noise reduction and stability. Since both op amps have wide bandwidths, the transformer limits the circuit's bandwidth. The measured noise of 3.1 dB is close to the calculated value of 3 dB and is a 3-dB improvement compared with using a single amplifier.

IC1 was selected on the basis of it low-voltage and current-noise values of 1.05 nV/Hz and 1.6 pA/Hz, respectively. IC2 was selected because it offers stable unity gain and a large full-signal-power bandwidth. Adding a 2.5-k Ohm resistor to IC1 lowers its quiescent current. (DI #1571)

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