CFA2

1
CFA2

• OA-13, AD_faq/ask2, OA-30, OA-07, OA-13

2
Understanding the loop gain OA-13

• CLC400 in nominal conditions
• RfRg250 W, Av2 Ri50 W
• Zt250502350 W 50.9 dB

3
(from OA13)
4
Controlling the loop gain

• If I need an Av different from the nominal one?
• I try to use the same feedback transimpedance Zt
• Using the nominal values, I compute Zt
• For the desired Av , I compute Rf
• and finally I get Rg from
• E.g, for the CLC406, Zt500606860 ohm

5
Controlling the loop gain

• This example for the CLC 404, whose nominal
  values are
• Av6
• Rf500 ohm
• Ri30 ohm
• so that
• Zt500306680 ohm
• (above) with fixed Rf
• (below) with Zt Zt better!

6
To drive a capacitive load AD_faq/ask2, p.2.9

• A capacitor on the output increases the phase -gt
  instability. The easiest solution
• - R in series

7
VFA / CFA comparison OA-30
8
VFA / CFA comparison OA-30

• Normally, the input stage of a VFB is a
  differential stage -gt two equally biased, matched
  transistors -gt error compensation -gt
• - low input offest voltage Vio
• - matched input bias current Ib
• - high PSRR
• - good CMRR
• Moreover, lower noise and feedback freedom

9
VFA / CFA comparison OA-30

• Normally, the CFA input stage is a push-pull
  stage -gt an NPN and a PNP -gt no matching -gt
• - higher Vio
• - unmatched Ib
• - worse PSRR and CMRR
• - higher noise limited feedback freedom

10
CFA

• No gain-bandwidth trade-off
• Often, the CFA is simpler (an input buffer, a
  gain stage and an output buffer) -gt fewer stages
  -gt faster
• I in C is not limited -gt no slew rate
  limitation, apart from
• - saturation effects AD_faq/ask1, p. 2.9
• - current gain degradation AN-NS92, p. 5.b.6
• - capacitive effects
• Some modern VFAs (e.g. AD9632) have now similar
  characteristics

11
CFA

• Distortions are normally due to asymmetries and
  limited speed
• - complementary symmetry -gt lower distortion
• - being faster, at high speed the loop gain is
  higher -gt lower distortion

12
Application circuit guide

• see OA-07
• E.g. high pass KRC!

13
CFA parameters modeled by the National SPICE
models OA-18

• DC EFFECTS
• VIO, IBI, IBN
• Supply current vs. supply voltages
• Common mode input/output voltage range
• Load current from supplies
• CMRR
• AC EFFECTS lt 500 MHz
• Frequency response vs. gain load
• Open loop gain phase
• Noise
• Small signal input/output impedance
• TIME DOMAIN
• Rise/fall times
• Slew rates
• SPECIAL FEATURES (WHERE APPLICABLE)
• Output clamping
• Supply current adjustment
• Offset voltage adjust

14
CFA parameters not modeled by the National
SPICE models

• Differential gain and phase
• PSRR
• Harmonic distortion
• Fine scale settling performance
• Thermal tail

15
A new CFA LMH6714

• Suggested operation AV2, Rf300 W
• see data sheets...
• Ri 180 W
• slew rate 1800 V/us
• improved replacement for CLC 400, 401, 402, 404,
  406
• suggested Rf values are provided for different
  values of AV (p. 9)
• for gains gt 5 it behaves as a GBP limited OA.