OPA820

+5V

−5V

50Ω

V

V

O

I

0.1µF

2.2µF

0.1µF

2.2µF

R

M

57.6Ω

R

T

205Ω

R

F

402Ω

50Ω Source

50Ω Load

0.01µF

R

G

402Ω

OPA820-HT

www.ti.com

SBOS587

– DECEMBER 2011

Wideband Inverting Operation

Operating the OPA820 as an inverting amplifier has several benefits and is particularly useful when a matched

50-

Ω source and input impedance is required. Figure 3 shows the inverting gain of −1 circuit used as the basis of

the inverting mode typical characteristics.

Figure 3. Inverting G =

−1 Specifications and Test Circuit

In the inverting case, just the feedback resistor appears as part of the total output load in parallel with the actual

load. For the 100-

Ω load used in the typical characteristics, this gives a total load of 80 Ω in this inverting

configuration. The gain resistor is set to get the desired gain (in this case 402

Ω for a gain of −1) while an

impedance of 50

Ω. This matching is only needed when the input needs to be matched to a source impedance,

as in the characterization testing done using the circuit of Figure 3.

The OPA820 offers extremely good DC accuracy as well as low noise and distortion. To take full advantage of

that DC precision, the total DC impedance looking out of each of the input nodes must be matched to get bias

current cancellation. For the circuit of Figure 32, this requires the 205-

Ω resistor shown to ground on the

noninverting input. The calculation for this resistor includes a DC-coupled 50-

Figure 3) to filter the noise contribution of the resistor and the input current noise.

exceed the bandwidth at that same gain magnitude for the noninverting circuit of Figure 2. This occurs due to the

lower noise gain for the circuit of Figure 3 when the 50-

Ω source impedance is included in the analysis. For

instance, at a signal gain of

be 1 + 499

Ω/(50 Ω + 50 Ω) = 6 as a result of adding the 50-Ω source in the noise gain equation. This gives

considerable higher bandwidth than the noninverting gain of +10. Using the 240-MHz gain bandwidth product for

the OPA820, an inverting gain of

noninverting gain of +10 gives 30 MHz.

Wideband Single-Supply Operation

Figure 4 shows the AC-coupled, single 5-V supply, gain of +2 V/V circuit configuration used as a basis for the

5 V 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 4 establishes an input midpoint bias using a simple resistive divider from the 5-V supply (two 806-

Ω

resistors) to the noninverting input. The input signal is then AC-coupled into this midpoint voltage bias. The input

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© 2011, Texas Instruments Incorporated

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Product Folder Link(s): OPA820-HT