Amplifier with negative feedback. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Amplifier with negative feedback. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment

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To increase the dynamic range of the amplifier, negative feedback (NFB) is used. Everyone knows the usual amplifier with transformer OOS, which is successfully used by many. Its only drawback is the low decoupling between the input and output: if the load is different from 50 ohms, then the input impedance is not equal to 50 ohms.

N6NWP described a high isolation amplifier that is used between a passive mixer and a crystal filter in a high dynamic receiver. However, it is rather complicated.

The amplifier described below is quite simple and has high dynamics, low noise figure, input impedance of 50 ohms, which depends little on the load impedance.

The decoupling between the input and the OOS circuit is based on an asymmetric power divider (Fig. 1).

Negative Feedback Amplifier
Ris.1

This divider has the following relations:

R2=R3(1+N)/N

R3=(l+N)Rs

R1=R2+R3

The input and output circuits are isolated from each other, and at point 1 the power is the sum of the power of point 2 and 1/N of the power of point 3. The amplifier circuit is shown in Fig.2.

Negative Feedback Amplifier
Ris.2

The following results are obtained:
- gain in the band 5...200 MHz - 10 dB;
- noise figure at 144 MHz - 2,5 ... ZdB;
- point IP3 - +14 dBm;
- decoupling between input and output:

f(MHz)	Output 50 ohm	Output not loaded
10	-15dB	-15dB
30	-32 dB	-30 dB
144	-6,5 dB	-4dB

I think that by improving the power divider at the input, one could achieve more constant performance over the entire range.

Literature

Radio Revistu, 11/1996

Author: P. Moroni (I5TDJ), translated by O. Popikova.; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Nodes of amateur radio equipment.

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