Electronic Design

Use Wideband Op Amps In Single-Supply Applications

Most wideband op amps are easiest to use with split supplies. But they can be effectively employed in single-supply circuits. The big hurdle to overcome is keeping the inputs and outputs biased to mid-rail while maintaining a low-impedance node to set the high frequency gain. This Design Brief shows how it's done and provides test data using the THS4302 op amp as an example.

This op amp uses internal feedback to fix the gain at 5 V/V. Also, the device features a 2.4-GHz bandwidth and is compensated to give maximum performance at this gain.

Most op amps operate best when the input and output signals are referenced to mid-rail. This seems a rather simple matter, but because of the part's high frequency of operation, stability is a concern. To remain stable, the amplifier's negative input must have a low-impedance reference (or ac ground) at very high frequency.

Use of a voltage reference or other voltage-regulating device to set the negative input at mid-rail is an obvious place to start. This will maintain the amplifier's gain at low frequencies. But at very high frequencies, a capacitor also must be utilized to keep the op amp stable.

This being the case, a resistive divider with bypass capacitor is a simpler solution for setting the dc bias and keeping the high-frequency gain and stability intact. The single-supply circuit shown in Figure 1 is applicable to dc-coupled systems if the input signal is biased to mid-rail. If this is the case, ac coupling the input isn't required.

The two 1-kΩ resistors set up a dc bias to place the input and output pins at mid-rail (VS/2). RIN and ROUT are used for termination. For proper impedance matching, they should equal the source or load resistance (whichever applies).

CIN and COUT block the dc level from input and output devices and preserve the dc bias for the amplifier. They form first-order high-pass filters on the input and output. The −3-dB cutoff frequency is calculated by:

f−-3dB = 1/2 π CREFF

where C is the input or output capacitor and REFF is the effective resistance seen in series with the capacitor. On the input, REFF equals the source resistance plus RIN, and on the output, the load resistance plus ROUT. If the system is 50 Ω and properly terminated, REFF = 100 Ω. High-frequency operation will be limited by the parasitic inductance of the input and output capacitors and the amplifier's frequency response.

To maintain stability, CN supplies a low impedance at high frequencies for the amplifier's negative input. CBYP is the power-supply bypass capacitor. It provides a local energy source for high-frequency operation. The circuit in Figure 1 is constructed with:

CIN, COUT, CN, and CBYP = 1000 pF
VS = +5 V
RIN and ROUT = 50 Ω

The gain, noise figure, and output third-order intercept measurements shown in Figure 2 were taken using equipment with 50-Ω I/O terminations. The results are identical to performance with ±2.5-V supplies.

Note that the measurement instrument's termination forms a resistive divider at the amplifier's output, thus affecting the gain and intercept figures. To refer the voltage gain to the amplifier's output, add 6 dB. To refer the IP3 to the amplifier's output, add 3 dB. This does not affect the noise figure.

As a further test, the bottom side 1-kΩ resistor in the voltage divider was replaced with a TL431 shunt regulator (set to 2.5 V), with the rest of the circuit unchanged. Test results were identical to those presented.

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