Amplifier with negative feedback

To increase the dynamic range of the amplifier uses negative feedback (EP). All known conventional amplifier with transformer feedback, which has been successfully used by many. Its only fault is a small isolation between input and output: if the load is different from 50 Ohms, and the impedance of the input is not equal to 50 Ohms.

N6NWP described amplifier with a good interchange, which is used between the passive mixer and the crystal filter in the receiver with high dynamics. However, it is quite complicated.

As described below, the amplifier is quite simple and has high dynamics, low noise figure, full electric input resistance of 50 Ohms, which is only slightly dependent on the load impedance.

Isolation between input and circuit EP is based on the asymmetric power divider (Fig.1).

Fig.1

This divider has the following relationship:

R2=R3(1+N)/N

R3=(l+N)Rs

R1=R2+R3

Chains input and output are isolated between them, and the point of power is 1 the amount of power points 2 and 1/N power point 3. The amplifier circuit shown in Fig.2.

Fig.2

The following results are obtained:
- the gain in the band 5...200 MHz - 10 dB;
is the coefficient of noise on 144 MHz to 2.5...SDB;
point IP3 is +14 dBm;
- isolation between input and output:

f(MHz)

Output 50 Ohm

The output is not loaded

10

-15dB

-15dB

30

-32 dB

-30 dB

144

-6,5 dB

-DB

I think that improving the power divider at the input, it would be possible to achieve a more permanent feature throughout the range.

Literature

Radio Revistu, 11/1996

Author: P. Moroni (I5TDJ), the translation of the O. Popkovoi.; Publication: N. Bolshakov, rf.atnn.ru