3-615
HFA1130
Application Information
Optimum Feedback Resistor (RF)
The enclosed plots of inverting and non-inverting frequency
response detail the performance of the HFA1100/1120 in
various gains. Although the bandwidth dependency on ACL
isn’t as severe as that of a voltage feedback amplifier, there is
an appreciable decrease in bandwidth at higher gains. This
decrease can be minimized by taking advantage of the
current feedback amplifier’s unique relationship between
bandwidth and RF. All current feedback amplifiers require a
f eedback resistor, even f or unity gain applications, and the RF,
in conjunction with the internal compensation capacitor, sets
the dominant pole of the frequency response. Thus, the
amplifier’s bandwidth is inversely proportional to RF. The
HFA1100, 1120 designs are optimized for a 510Ω RF, at a
gain of +1. Decreasing RF in a unity gain application
decreases stability, resulting in excessive peaking and
overshoot (Note: Capacitive feedback causes the same
problems due to the feedback impedance decrease at higher
frequencies). At higher gains the amplifier is more stable, so
RF can be decreased in a trade-off of stability for bandwidth.
The table below lists recommended RF values for various
gains, and the expected bandwidth.
Clamp Operation
General
The HFA1130 features user programmable output clamps to
limit output voltage excursions. Clamping action is obtained
by applying voltages to the VH and VL terminals (pins 8 and
5) of the amplifier. VH sets the upper output limit, while VL
sets the lower clamp level. If the amplifier tries to drive the
output above VH, or below VL, the clamp circuitry limits the
output voltage at VH or VL (± the clamp accuracy), respec-
tively. The low input bias currents of the clamp pins allow
them to be driven by simple resistive divider circuits, or
active elements such as amplifiers or DACs.
Clamp Circuitry
Figure 1 shows a simplified schematic of the HFA1130 input
stage, and the high clamp (VH) circuitry. As with all current feed-
back amplifiers, there is a unity gain buffer (QX1 - QX2) between
the positive and negative inputs. This buffer forces -IN to track
+IN, and sets up a slewing current of (V-IN -V
OUT)/RF. This cur-
rent is mirrored onto the high impedance node (Z) by QX3-QX4,
where it is converted to a voltage and fed to the output via
another unity gain buffer. If no clamping is utilized, the high
impedance node ma y swing within the limits defined by QP4 and
QN4. Note that when the output reaches it’s quiescent value, the
current flowing through -IN is reduced to only that small current
(-IBIAS) required to keep the output at the final v oltage .
Tracing the path from VH to Z illustrates the effect of the clamp
voltage on the high impedance node. VH decreases by 2VBE
(QN6 and QP6) to set up the base voltage on QP5. QP5 begins
to conduct whenever the high impedance node reaches a volt-
age equal to QP5’s base + 2VBE (QP5 and QN5). Thus, QP5
clamps node Z whenever Z reaches VH. R1 provides a pull-up
network to ensure functionality with the clamp inputs floating. A
similar description applies to the symmetrical low clamp cir-
cuitry controlled b y VL.
When the output is clamped, the negative input continues to
source a slewing current (ICLAMP) in an attempt to force the out-
put to the quiescent voltage defined b y the input. QP5 must sink
this current while clamping, because the -IN current is always
mirrored onto the high impedance node. The clamping current
is calculated as (V-IN - VOUT)/RF. As an example, a unity gain
circuit with VIN = 2V, VH = 1V, and RF = 510Ω would have
ICLAMP = (2-1)/510Ω = 1.96mA. Note that ICC will increase by
ICLAMP when the output is clamp limited.
Clamp Accuracy
The clamped output voltage will not be exactly equal to the volt-
age applied to VH or VL. Offset errors, mostly due to VBE mis-
matches, necessitate a clamp accuracy parameter which is
f ound in the device specifications. Clamp accur acy is a function
of the clamping conditions. Referring again to Figure 1, it can
be seen that one component of clamp accuracy is the VBE mis-
match between the QX6 transistors, and the QX5 transistors. If
the transistors always ran at the same current level there would
be no VBE mismatch, and no contribution to the inaccur acy. The
QX6 transistors are biased at a constant current, but as
described earlier, the current through QX5 is equivalent to
ICLAMP. VBE increases as ICLAMP increases, causing the
clamped output voltage to increase as w ell. ICLAMP is a function
of the overdrive level (V-IN -VOUTCLAMPED) and RF, so clamp
accuracy degrades as the overdrive increases, or as RF
decreases. As an example, the specified accuracy of ±60mV
for a 2X overdrive with RF= 510Ω degrades to ±220mV for
RF= 240Ω at the same overdrive, or to ±250mV for a 3X
ov erdriv e with RF = 510Ω.
ACL RF(Ω) BW (MHz)
+1 510 850
-1 430 580
+2 360 670
+5 150 520
+10 180 240
+19 270 125
+1
+IN V-
V+
QP1
QN1
V-
QN3
QP3 QP4
QN2
QP2
QN4 QP5
QN5
Z
V+
-IN VOUT
ICLAMP
RF
(EXTERNAL)
QP6
QN6
VH
R1
50K
(30K
FOR VL)
200Ω
FIGURE 1. HFA1130 SIMPLIFIED VH CLAMP CIRCUITRY