Datasheet

V =R I´
IN(PP) RG(PP)G
R =
GMIN
=615.4W
3.2V
PP
5.2mA
PP
VCA820
www.ti.com ............................................................................................................................................ SBOS395C –OCTOBER 2007–REVISED OCTOBER 2009
OUTPUT VOLTAGE DYNAMIC RANGE
The minimum specified output voltage and current
over-temperature are set by worst-case simulations at
With its large output current capability and its wide
the cold temperature extreme. Only at cold startup do
output voltage swing of ±3.9V typical on 100Ω load, it
the output current and voltage decrease to the
is easy to forget other types of limitations that the
numbers shown in the Electrical Characteristics
VCA820 can encounter. For these limitations, careful
tables. As the output transistors deliver power, the
analysis must be done to avoid input stage limitation,
respective junction temperatures increase, increasing
either voltage or I
RG
current; also, consider the gain
the available output voltage swing, and increasing the
limitation, as the control pin V
G
varies, affecting other
available output current. In steady-state operation,
aspects of the circuit.
the available output voltage and current is always
greater than that temperature shown in the
BANDWIDTH
over-temperature specifications because the output
stage junction temperatures are higher than the The output stage of the VCA820 is a wideband
specified operating ambient. current-feedback amplifier. As such, the feedback
resistance is the compensation of the last stage.
INPUT VOLTAGE DYNAMIC RANGE Reducing the feedback element and maintaining the
gain constant limits the useful range of I
RG
, and
The VCA820 has a input dynamic range limited to
therefore reducing the gain adjust range. For a given
+1.6V and –2.1V. Increasing the input voltage
gain, reducing the gain element limits the maximum
dynamic range can be done by using an attenuator
achievable output voltage swing.
network on the input. If the VCA820 is trying to
regulate the amplitude at the output, such as in an
OFFSET ADJUSTMENT
AGC application, the input voltage dynamic range is
directly proportional to Equation 3. As a result of the internal architecture used on the
VCA820, the output offset voltage originates from the
(3)
output stage and from the input stage and multiplier
core. Figure 88 illustrates how to compensate both
As such, for unity-gain or under-attenuated
sources of the output offset voltage. Use this
conditions, the input voltage must be limited to the
procedure to compensate the output offset voltage:
CMIR of ±1.6V (3.2V
PP
) and the current (I
RQ
) must
starting with the output stage compensation, set
flow through the gain resistor, ±2.6mA (5.2mA
PP
).
V
G
= 0V to eliminate all offset contribution of the input
This configuration sets a minimum value for R
E
such
stage and multiplier core. Adjust the output stage
that the gain resistor has to be greater than
offset compensation potentiometer. Finally, set
Equation 4.
V
G
= +1V to the maximum gain and adjust the input
stage and multiplier core potentiometer. This
procedure effectively eliminates all offset contribution
(4)
at the maximum gain. Because adjusting the gain
Values lower than 615.4Ω are gain elements that
modifies the contribution of the input stage and the
result in reduced input range, as the dynamic input
multiplier core, some residual output offset voltage
range is limited by the current flowing through the
remains.
gain resistor R
G
(I
RG
). If the I
RG
current is limiting the
performance of the circuit, the input stage of the
VCA820 goes into overdrive, resulting in limited
output voltage range. Such I
RG
-limited overdrive
conditions are shown in Figure 52 for the gain of
20dB and Figure 72 for the 40dB gain.
Copyright © 2007–2009, Texas Instruments Incorporated Submit Documentation Feedback 25
Product Folder Link(s): VCA820