RHF43B_11 Datasheet, PDF (11/16 Pages) STMicroelectronics – Rad-hard precision bipolar single operational amplifier

RHF43B
Achieving good stability at low gains
3
Achieving good stability at low gains
At low frequencies, the RHF43B can be used in a low gain configuration as shown in
Figure 23. 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).
Figure 23. Low gain configuration
V_CC
+
Vin
-
VDD
C
Rfb = 2 kΩ
Rg
R
C_L = 100 pF
Figure 24. Closed-loop gain
Vout
Gain
(dB)
Frequencies
where the
op-amp can
be used
A _VD
1
2πRC
G2=1+Rfb/(Rg//R)
RL
1 kΩ
+20 dB/dec
-20 dB/dec
G1=1+Rfb//Rg
0 dB
G1
2 π(G1R+Rfb)C
Gain bandwidth
product
Bandwidth Log frequency
of the
op-amp at G2
AM06122
AM06123
Rg becomes a complex impedance. The closed-loop gain features a variation in frequency
and can be expressed as:
1
+
jCω
×
⎛
⎝
G_------1_---R_{--G----+-1----}R_-----f_--b_{- ⎠⎞}
Gain = G1-------------------₁-----+-----_j-_C-----_R-----_ω---------------------
where a pole appears at 1/2πRC and a zero at G1/2π(G1R+Rfb)C. The frequency can be
plotted as shown in Figure 24.
Table 5. External components versus low-frequency gain
G1 (V/V)
R (Ω)
C (nF)
Rg (Ω)
1.1
510
1
20k
2
510
1
2k
3
510
1
1k
4
510
1
750
5
Not connected Not connected
820
Rfb (Ω)
2k
2k
2k
2.4k
3.3k
Doc ID 13477 Rev 8
11/16