RHF484

Achieving good stability at low gain

Doc ID 17351 Rev 1

13/18

3

Achieving good stability at low gain

At low frequencies, the RHF484 can be used in a low gain configuration as shown in

Figure 24. At lower frequencies, the stability is not affected by the value of the gain, which

can be set close to 1 V/V (0 dB), and is reduced to its simplest expression G1=1+Rfb/Rg.

Therefore, an R-C cell is added in the gain network so that the gain is increased (up to 5) at

higher frequencies (where the stability of the amplifier could be affected). At higher

frequencies, the gain becomes G2=1+Rfb/(Rg//R).

Rg becomes a complex impedance. The closed-loop gain features a variation in frequency

and can be expressed as:

where a pole appears at 1/2

πRC and a zero at G1/2π(G1R+Rfb)C. The frequency can be

plotted as shown in Figure 25.

Figure 24.

Low gain configuration

Figure 25.

Closed-loop gain

Table 6.

External components versus low-frequency gain

G1 (V/V)

R (

Ω)C (nF)

Rg (

Ω)Rfb (Ω)

1.1

510

1

20k

2k

2510

1

2k

2k

3510

1

1k

2k

4

510

1

750

2.4k

5

Not connected

Not connected

820

3.3k

Gain

G1

1jC

ω

G1R

Rfb

+

G1

-----------------------------

⎝⎠

⎛⎞

×

+

1

jCR

ω

+

-------------------------------------------------------------

=