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Current feedback op-amps
References: National Semiconductor application notes: OA15, AN-597Texas Instruments: OpAmps for Everyone, Chapter 8: CFOAIntersil Elantec EL-5166 datasheet
What is a CFOA?
A CCII- , a voltage follower, and a node impedance
How did the idea come about?
Supply mirroring: CC Supply mirroring + Buffer
Current conveyor Current feedback op-amp
Loop gain calculation
Basic usage: Non inverting amplifier
( )F GOUTNINV
IN G F B G B F G
Z Z ZdVGdV ZZ Z Z Z Z Z Z
+= =
+ + +
Note that we have not introduced explicit frequency dependence!
lim 1 FNINVZ
G
ZGZ→∞
= +
Not common usage: Inverting
GINV −ZZFZZGZBZFZBZGZFZG
lim FINVZ
G
ZGZ→∞
= −
Gain calculation – frequency response
GNINV ZZFZZGZZGZBZFZBZGZFZG
ZT Z0
1s1 1s2
ZB hib RB01
1s0T1sT /11/0
Transimpedance ZT has two poles: due to the impedance of the high Z nodeand also due to the current mirror delay ZB is the output impedance of the input buffer, small but not necessarily much smaller than ZG
Gain calculation – frequency response (2)
GNINV ZZFZZGZZGZBZFZBZGZFZG ZT
Z01s1 1s2
Neglect ZB since it is much smaller than anything else. Then,
( ) 111 /
F G FNINV
G F G G F
BandwidthGain
Z Z Z ZZZZ Z Z Z Z Z
+ ⎛ ⎞= +⎜ ⎟+ +⎝ ⎠
Observe that the gain and bandwidth have been decoupled. The bandwidth depends only on ZF (and ZB, really), while the gain depends on both ZF and ZG
There is no magic involved. The CFOA is a dominant pole transimpedance amplifier, while the voltage and current buffers have poles at much higher frequencies.
Gain vs. frequency with RF (EL5166)
Gain vs frequency with RG (EL5166)
Notice that despite what our approximations say the BW still depends on gain!
Gain bandwidth product – example
Frequency response of transimpedance
The transimpedance has a dominant pole at 100kHz
Slew rate: VFOA vs CFOA
• VFOA:
• CFOA:
0
0.5
1
1.5
2
2.5
3
3.5
4
0 2 4 6 8 10
In the limit of large input voltages,in the VFOA:
While in the CFOA we get:
The slew rate in the CFOA is power limited
0out
in
dIdV
→
outout
in
dI IdV
→∝
( )tanhoutVFOAI V V+ −∝ −
( )expoutCFOA yI V∝
Re-visit the non inverting amplifier
The CFOA is a transimpedance amplifier, i.e. equivalent to the block diagramThe response is, in terms of the transimpedance Z is the usual FB expression:
VIN
YG+YF Z
YF
VOUT
( ) lim 1 11
G F G FNINV NINVZ
F F G
Z Y Y Y ZG GZY Y Z→∞
+= ⇒ = + = +
+Notice that the loop gain is ZZF and that Z is typically second order beyond the first pole, so that, for a parallel RC in the feedback path we get a second order underdamped system.
Avoid capacitances!
Effect of Feedback capacitance
INSTABILITY unless we introduce extra pole!
Input capacitance
Brute force compensation
Low pass the non-inverting input to compensate for stray capacitance,i.e. brute force compensation
Effect of input (stray) capacitance
Output (load) capacitance
Applications I
Inverting receiver Non-inverting receiver
Applications II
Differential amplifier Differential line driver
Antenna circuits
Coax cable driver Distribution amplifier
CFOA can be combined with other stages…
With emitter follower
With op-amps
Bandwidth and stability
Adjust BW by increasing inverting input impedance!
Integrator
Integrating capacitance NOT in feedback path. Capacitance must be lossy enough!
Implementation note: Supply bypassing is NOT optional!