Current feedback op-amps

Current feedback op-amps
References:
National Semiconductor application notes: OA15, AN-597
Texas Instruments: OpAmps for Everyone, Chapter 8: CFOA
Intersil 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
G
NINV
dV
OUT
= =
ZF
lim GNINV
= 1+
Z →∞ Z
( + Z )
Z Z
dV ZZ + Z Z + Z Z + Z Z
IN G F B G B F G
G
Note that we have not introduced explicit frequency dependence!
F
G

Not common usage: Inverting
G INV
−ZZ F
ZZ G Z B Z F Z B Z G Z F Z G
lim
Z →∞
G
INV
=−
Z
Z
F
G

Gain calculation – frequency response
G NINV
ZZ F ZZ G
ZZ G Z B Z F Z B Z G Z F Z G
Z B h ib
R B
0 1
1s 0 T
1s T
/11/ 0
Z T
Z 0
1s 1 1s 2
Transimpedance Z T
has two poles: due to the impedance of the high Z node
and also due to the current mirror delay
Z B
is the output impedance of the input buffer, small but not necessarily much
smaller than Z G

Gain calculation – frequency response (2)
G NINV
ZZ F ZZ G
ZZ G Z B Z F Z B Z G Z F Z G
Z T
Z 0
1s 1 1s 2
Neglect Z B
since it is much smaller than anything else. Then,
Z
NINV
( + ) ⎛ ⎞ 1
Z ZF ZG ZF
= ⎜1+
⎟
ZZG + ZFZG ⎝ ZG ⎠1 + ZF
/ Z
Gain
Bandwidth
Observe that the gain and bandwidth have been decoupled.
The bandwidth depends only on Z F
(and Z B
, really), while the
gain depends on both Z F
and Z G
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 R F (EL5166)

Gain vs frequency with R G (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:
IoutVFOA
∝tanh
( V
+
−V−
)
IoutCFOA
∝ exp( Vy
)
dIout
0
dV
→
in
dIout
Iout
dV →∝
In the limit of large input voltages,
in the VFOA:
While in the CFOA we get:
in
4
3.5
3
2.5
2
1.5
1
0.5
0
0 2 4 6 8 10
The slew rate in the CFOA is power limited

Re-visit the non inverting amplifier
V IN
Y G
+Y F
Z
V OUT
Y F
The CFOA is a transimpedance amplifier, i.e. equivalent to the block diagram
The response is, in terms of the transimpedance Z is the usual FB expression:
G
NINV
( + )
Z Y Y Y Z
= ⇒ lim GNINV
= 1+ = 1+
1+
ZY Z →∞ Y Z
G F G F
F F G
Notice that the loop gain is ZZ F
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

Adjust BW by increasing inverting
input impedance!
Bandwidth and stability

Integrator
Integrating capacitance NOT in
feedback path.
Capacitance must be lossy enough!

Implementation note:
Supply bypassing is NOT optional!