Datasheet

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SBOS303C − JUNE 2004 − REVISED AUGUST 2008
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14
WIDEBAND SINGLE-SUPPLY OPERATION
Figure 3 shows the AC-coupled, single +5V supply, gain of
+2V/V circuit configuration used as a basis for the +5V only
Electrical and Typical Characteristics. The key requirement for
single-supply operation is to maintain input and output signal
swings within the useable voltage ranges at both the input and
the output. The circuit of Figure 3 establishes an input
midpoint bias using a simple resistive divider from the +5V
supply (two 806Ω resistors) to the noninverting input. The
input signal is then AC-coupled into this midpoint voltage bias.
The input voltage can swing to within 0.9V of the negative
supply and 0.5V of the positive supply, giving a 3.6V
PP
input
signal range. The input impedance matching resistor (57.6Ω)
used in Figure 3 is adjusted to give a 50Ω input match when
the parallel combination of the biasing divider network is
included. The gain resistor (R
G
) is AC-coupled, giving the
circuit a DC gain of +1. This puts the input DC bias voltage
(2.5V) on the output as well. On a single +5V supply, the output
voltage can swing to within 1.3V of either supply pin while
delivering more than 80mA output current giving 2.4V output
swing into 100Ω (5.6dBm maximum at the matched load).
Figure 4 shows the AC-coupled, single +5V supply, gain of
−1V/V circuit configuration used as a basis for the +5V only
Typical Characteristic curves. In this case, the midpoint DC
bias on the noninverting input is also decoupled with an
additional 0.01µF decoupling capacitor. This reduces the
source impedance at higher frequencies for the noninverting
input bias current noise. This 2.5V bias on the noninverting
input pin appears on the inverting input pin and, since R
G
is DC
blocked by the input capacitor, will also appear at the output
pin.
The single-supply test circuits of Figure 3 and Figure 4 show
+5V operation. These same circuits can be used over a single-
supply range of +5V to +12V. Operating on a single +12V
supply, with the Absolute Maximum Supply voltage
specification of +13V, gives adequate design margin for the
typical ±5% supply tolerance.
OPA820
+5V
+V
S
DIS
100
Ω
R
G
402
Ω
806
Ω
806
Ω
57.6
Ω
0.01
µ
F
+
6.8
µ
F0.1
µ
F
0.01
µ
F
V
I
50
Ω
Source
R
F
402
Ω
V
O
V
S
/2
Figure 3. AC-Coupled, G = +2V/V, Single-Supply Specifications and Test Circuit
OPA820
+5V
+V
S
DIS
100
Ω
R
G
402
Ω
806
Ω
806
Ω
0.01
µ
F
0.01
µ
F
V
I
R
F
402
Ω
V
O
V
S
/2
+
6.8
µ
F0.1
µ
F
Figure 4. AC-Coupled, G = −1V/V, Single-Supply Specifications and Test Circuit