10.1.1 Lowering Offset Voltage and Drift

The OPA454 can be used with an OPA735 zero-drift series op amp to create a high-voltage op amp circuit that has very low input offset temperature drift. This circuit is shown in Figure 64.

Figure 64. Two-Stage, High-Voltage Op Amp Circuit with Very Low Input Offset Temperature Drift

10.1.2 Increasing Output Current

The OPA454 drives an output current of a few milliamps to greater than 50 mA while maintaining good op amp performance. See Figure 6 for open-loop gain versus temperature at various output current levels.

In applications where the 25-mA output current is not sufficient to drive the required load, the output current can be increased by connecting two or more OPA454s in parallel, as Figure 65 shows. Amplifier A1 is the master amplifier and may be configured in virtually any op amp circuit. Amplifier A2, the slave, is configured as a unity-gain buffer. Alternatively, external output transistors can be used to boost output current. The circuit in Figure 66 is capable of supplying output currents up to 1 A, with the transistors shown.

1.

NOINDENT:

R_S resistors minimize the circulating current that always flows between the two devices because of V_OS errors.

Figure 65. Parallel Amplifiers Increase Output Current Capability

1.

NOINDENT:

Provides current limit for OPA454 and allows the amplifier to drive the load when the output is between +0.7 V and –0.7 V.

2.

NOINDENT:

Op amp V_OUT swings from +47 V to –48 V.

3.

NOINDENT:

V_O swings from +44.1 V to –45.1 V at I_L = 1 A.

Figure 66. External Output Transistors Boost Output Current Greater Than 1 A

10.1.3 Unity-Gain Noninverting Configuration

When in the noninverting unity-gain configuration, the OPA454 has more gain peaking with increasing positive common-mode voltage and increasing temperature. It has less gain peaking with more negative common-mode voltage. As with all op amps, gain peaking increases with increasing capacitive load. A resistor and small capacitor placed in the feedback path can reduce gain peaking and increase stability.

10.2.1 Design Requirements

Figure 67 shows an output voltage boost circuit where three OPA454 op amps connected as shown can produce an output voltage swing as high as 195 V_PP. The resulting output swing range is twice that attainable with a single OPA454 device operating from ±50-V supplies, and is useful in applications where an even higher output swing is required. A ±100-V_DC power supply is required for this configuration.

Three of the design goals for this circuit are:

• A noninverting gain of 20 V/V (26 dB)
• A peak output voltage approaching 97.5 V, while delivering a peak output current of approximately 24 mA
• Correct biasing of the Enable/Disable (E/D), E/D Com, and Status flag pins

10.2.2 Detailed Design Procedure

U3 (an OPA454) is the only amplifier of the three devices in the application that is responsible for signal amplification. The other two op amps, U1 and U2, provide the positive and negative supply sources (respectively) for U3. The voltage gain of U3 is that of a traditional noninverting op amp amplifier. A simple relation involving U3 feedback (R₂) and input (R₁) resistors sets the closed-loop gain, A_V. Equation 1 shows this calculation.

Equation 1.

where

• R₁ = 10 kΩ
• R₂ = 190 kΩ
• A_V = 20 V/V

Applying this gain and a V_PK of ±97.5 V, the maximum input voltage that may be applied without causing the output to clip is ±4.75 V.

U1 and U2 are connected as unity-gain buffers. The purpose of this configuration is to track the U3 output voltage, and then adjust the voltage levels at the U3 V+ and V– pins so that 100 V is maintained across them. This input is accomplished by the U1 and U2 input connection to the U3 output voltage through the 100-kΩ voltage dividers formed by R₁₁, R₁₂, and R₁₄, R₁₅. For example, as the output of U3 moves more positive, the voltage on the U1 noninverting input moves up more closely to the +100-V supply level. Even though U2 provides the U1 V– supply, its output moves more positive as well. The result is that all the devices move together in unison up and down, while maintaining the 100-V difference between the V+ and V– pins for U3.

Figure 68 shows how the U3 op amp output voltage V_LOAD moves upward, becoming more positive, as the input voltage VG1 increases from –4.75 V to 4.75 V. Figure 68 also shows V_O1 and V_O2, the U1 and U2 (respectively) output voltages. The 100-V difference between the supply pins is evident in the graph. Notice how the U3 V+ pin (V_O1) allows 100 V greater than its V– pin (V_O2).

Figure 68. OPA454 Output Voltage Levels vs V_S1 Input Voltage

Figure 69 shows the V_LOAD output, a 195-V_PP, 20-kHz sine wave, as developed across the 3.75-kΩ load resistor. The peak current provided by the OPA454 U3 output is 26 mA. U1 and U2 alternately source and sink the output current, in addition to the operating current required by U3.

Figure 69. Output Voltage Boost Develops 195-V_PP V_LOAD Across 3.75-kΩ Load Resistance

The output voltage booster may be used in an inverting configuration also. This use is easily accomplished by applying the input signal to the input resistor R₁ as seen in Figure 70. The noninverting input is grounded and the ratio of feedback resistor R₂ to R₁ is set to 20:1 to satisfy the inverting gain equation given in Equation 2.

Equation 2.

where

• R₁ = 10 kΩ
• R₂ = 200 kΩ
• A_V = –20 V/V

Figure 70. OPA454 Output Boost Circuit Applied as an Inverting Amplifier

10.2.3 Application Curve

Figure 71 shows an example of the inverting output boost amplifier output waveforms obtained from a TINA-TI simulation.

Figure 71. Voltage Levels in OPA454 Inverting Boost Amplifier Circuit from TINA-TI Simulation

10.3.1 Basic Noninverting Amplifier

Figure 72 shows the OPA454 connected as a basic noninverting amplifier. The OPA454 can be used in virtually any ±5-V to ±50-V op amp configuration. It is especially useful for supply voltages greater than 36 V.

Power-supply terminals must be bypassed with 0.1-μF (or greater) capacitors, located near the power-supply pins. Be sure that the capacitors are appropriately rated for the power-supply voltage used.

1. Pullup resistor with at least 10 μA (choose R_P = 1 MΩ with V+ = 50 V for I_P = 50 μA).

Figure 72. Basic Noninverting Amplifier Configuration

10.3.2 Programmable Voltage Source

Figure 73 illustrates the OPA454 in a programmable voltage source.

Figure 73. Programmable Voltage Source

10.3.3 Bridge Circuit

Figure 74 shows the OPA454 in a bridge circuit.

1.

NOINDENT:

For transducers with large capacitance, stabilization may become an issue. Be certain that the Master amplifier is stable before stabilizing the Slave amplifier.

Figure 74. Bridge Circuit Doubles Voltage for Exciting Piezo Crystals

10.3.4 High-Compliance Voltage Current Sources

This section describes four different applications using high-compliance voltage current sources with differential inputs. Figure 75 shows a high-voltage difference amplifier circuit. Figure 76 and Figure 78 illustrate the different applications.

Figure 75. High-Voltage Difference Amplifier

Figure 76. Differential Input Voltage-to-Current Converter for Low I_OUT

A red light emitting diode (LED) was used to generate Figure 77.

Figure 77. Avalanche Photodiode Circuit

Gain of the avalanche photodiode (APD) is adjusted by changing the voltage across the APD. Gain starts to increase when reverse voltage is increased beyond 130 V for this APD diode. See Figure 78.

Figure 78. APD Gain Adjustment Using the OPA454, High-Voltage Op Amp

10.3.5 High-Voltage Instrumentation Amplifier

Figure 79 uses three OPA454s to create a high-voltage instrumentation amplifier. V_CM ± V_SIG must be between (V–) + 2.5 V and (V+) – 2.5 V. The maximum supply voltage equals ±50 V or 100 V total.

1.

NOINDENT:

The linear input range is limited by the output swing on the input amplifiers, A₁ and A₂.

Figure 79. High-Voltage Instrumentation Amplifier

Figure 80 uses three OPA454s to measure current in a high-side shunt application. V_SUPPLY must be greater than V_CM. V_CM must be between (V–) + 2.5 V and (V+) – 2.5 V. Adhering to these restrictions keeps V₁ and V₂ within the voltage range required for linear operation of the OPA454. For example, if V+ = 50 V and V– = 50 V, then
V₁ = +47.5 V (maximum) and V₂ = –47.5 V (minimum). The maximum supply voltage equals ±50 V, or 100 V total.

1.

NOINDENT:

To increase the linear input voltage range, configure A₁ and A₂ as unity-gain followers.

2.

NOINDENT:

The linear input range is limited by the output swing on the input amplifiers, A₁ and A₂.

Figure 80. High-Voltage Instrumentation Amplifier for Measuring High-Side Shunt

Figure 81 shows an example circuit that uses the OPA454 in an output voltage boost configuration with six op amp output stages.

Figure 81. Output Voltage Boost with ±195 V (390 V_PP) Across Bridge-Tied Load
(Six Op Amps, see Figure 82 and Figure 83)

Figure 82. 390 V_PP Across 7.5-kΩ Load
20 kHz, Uses Six OPA454s, 100-V Supplies

SR of 34 V/ms, which is significantly higher than the specified 13 V/ms due to tracking of the power-supply voltage

Figure 83. 7.5-kΩ Load, G = +20, Six OPA454s, 100-V Supplies