OPA2634

13

SBOS098A

INVERTING AMPLIFIER OPERATION

Since the OPA2634 is a general-purpose, wideband voltage-

feedback op amp, all of the familiar op amp application

circuits are available to the designer. Figure 7 shows a

typical inverting configuration where the I/O impedances

and signal gain from Figure 1 are retained in an inverting

circuit configuration. Inverting operation is one of the more

common requirements and offers several performance ben-

efits. The inverting configuration shows improved slew rate

headroom. The output voltage can be independently moved

with bias-adjustment resistors connected to the inverting input.

The second major consideration, touched on in the previous

paragraph, is that the signal source impedance becomes

part of the noise gain equation and hence, influences the

combines in parallel with the external 50

Ω source imped-

ance, yielding an effective driving impedance of

50

Ω || 57.6Ω = 26.8Ω. This impedance is added in series

as discussed above. The bandwidth will, therefore, be lower

for the gain of –2 circuit of Figure 7 (NG = +2.87) than for

the gain of +2 circuit of Figure 1.

The third important consideration in inverting amplifier design

is setting the bias current cancellation resistors on the non-

resistor is set equal to the total DC resistance looking out of the

inverting node, the output DC error, due to the input bias

of the 0.1

µF capacitor, the inverting input’s bias current flows

the minimum output offset voltage. To reduce the additional

feedthrough, it is bypassed with a 0.1

µF capacitor. At a

minimum, the OPA2634 should see a source resistance of at

least 50

Ω to damp out parasitic-induced peaking—a direct

short to ground on the non-inverting input runs the risk of a very

high-frequency instability in the input stage.

OUTPUT CURRENT AND VOLTAGE

The OPA2634 provides outstanding output voltage capabil-

ity. Under no-load conditions at +25

°C, the output voltage

typically swings closer than 140mV to either supply rail; the

guaranteed over temperature swing is within 300mV of

The minimum specified output voltage and current specifi-

cations over temperature are set by worst-case simulations at

the cold temperature extreme. Only at cold start-up will the

output current and voltage decrease to the numbers shown in

the guaranteed tables. As the output transistors deliver power,

their junction temperatures will increase, decreasing their

increasing their current gains (increasing the available out-

put current). In steady-state operation, the available output

voltage and current will always be greater than that shown

in the over-temperature specifications, since the output stage

junction temperatures will be higher than the minimum

specified operating ambient.

To maintain maximum output stage linearity, no output

short-circuit protection is provided. This will not normally

be a problem, since most applications include a series

matching resistor at the output that will limit the internal

power dissipation if the output side of this resistor is shorted

to ground.

FIGURE 7. Gain of –2 Example Circuit.

50

Ω

R

F

750

Ω

R

G

374

Ω

R

M

57.6

Ω

Source

+5V

2R

T

1.50k

Ω

2R

T

1.50k

Ω

R

O

50

Ω

0.1

µF

6.8

µF

+

0.1

µF

0.1

µF

50

Ω Load

1/2

OPA2634

In the inverting configuration, three key design consider-

becomes part of the signal channel input impedance. If input

impedance matching is desired (which is beneficial when-

ever the signal is coupled through a cable, twisted pair, long

adjusted to give the desired gain. This is the simplest

approach and results in optimum bandwidth and noise per-

formance. However, at low inverting gains, the resultant

feedback resistor value can present a significant load to the

50

Ω for input matching eliminates the need for R

requires a 100

Ω feedback resistor. This has the interesting

advantage of the noise gain becoming equal to 2 for a 50

Ω

source impedance—the same as the non-inverting circuits

considered above. However, the amplifier output will now

see the 100

Ω feedback resistor in parallel with the external

load. In general, the feedback resistor should be limited to

the 200

Ω to 1.5kΩ range. In this case, it is preferable to

and then achieve the input matching impedance with a third