Optimizing Dual Feedback Compensation in the OPA593 With a Current Booster for 1μF Capacitive Loads in ATE Applications

Abstract

Since the introduction of the OPA593, the power operational amplifier (PA) has gained traction in the test and measurement sector. Specifically designed for Automated Test Equipment (ATE) applications, the OPA593 can drive output voltages up to 80V and output currents up to ±250mA, all within a compact 4mm × 4mm WSON package. The OPA593 operates across the full industrial temperature range of -40°C to 125°C, providing exceptional DC precision and robust output current limiting features that cater to diverse design requirements in ATE applications.

This application note demonstrates how to compensate for the OPA593 PA and current booster configuration, enabling output driving currents up to ±1A. Additionally, the document explains the implementation of the op amp’s dual feedback compensation techniques when driving capacitive loads of up to 1μF, making sure of adequate phase margin, stabilizing loop gains through AC analysis, and achieving fast step time responses in ATE applications.

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1 Introduction

The OPA593 is a high-voltage, high-output-current power amplifier (PA) that operates on an 85V single supply or a ±42.5V dual-supply configuration, with the capability to source or sink current up to ±250mA. This article focuses on dual-supply rail configurations, favored for their programmable and flexible precision voltage regulator setups commonly used in automated test equipment (ATE) applications. While the OPA593 meets the output voltage requirements for most power voltage regulator applications, OPA593 does not provide sufficient current drive in certain scenarios. In such cases, combining the OPA593 with a current booster topology can enhance the current drive capabilities while maintaining the amplifier’s overall operating voltage range, bandwidth, accuracy, and responsiveness to timing requirements.

In practice, a large capacitive load is often connected to the output of a power amplifier stage. Capacitive loads serve several purposes, including decoupling, filtering high-frequency noise, reducing voltage spikes, stabilizing transient responses, and improving output voltage regulation at the device under test (DUT). However, adding capacitive loads can introduce undesirable phase lag, potentially leading to loop instability in the power amplifier's feedback system.

Driving large capacitive loads presents significant design challenges for engineers, particularly in compensating for stability issues. This application note addresses these challenges when using the OPA593 with a Darlington current booster configuration. The document also explores the trade-offs associated with this technique, especially when driving capacitive loads up to 1µF.

Figure 1-1 and Table 2-2 present the schematic discussed in this application note, which aims to meet (or exceed) the design requirements.

Figure 1-1 The OPA593 With a Current Booster Circuit Drives 1μF Capacitive Load, C_L

Table 1-1 ATE Design Requirements for OPA593 + Current Booster

Design Parameters	Composite Amplifier's Voltage Regulator Specifications
Input voltage range	Input swing up to ±5V_dc
Output voltage range	Output swing up to ±40V_dc
Output current range	OPA593 with current booster, driving up to ±1A_dc
Output impedance	R_L ≥ 40Ω
Closed-loop gains	8V/V
Open-loop output impedance	Open loop output impedance, Z_o < approximately 1Ω
Capacitive load	Low ESR (20mΩ), 1µF ceramic capacitive load and DUT
Effective bandwidth	Approximately 50kHz, cutoff frequency at the –3dB point
Step time behavior	Output rising/falling edge step-time response <100µs
Output voltage accuracy	Approximately 0.05% or better across full scale

2 Current Booster, Push-Pull Topology Output Characteristics

The current booster’s open-loop output impedance and frequency response are among the key parameters in selecting a driver for ATE applications. The following criteria are summarized.

Table 2-1 Current Booster Driver Selection Criteria

No	Current Booster Driver Selection Guide
1	Low and consistent open-loop output impedance, with Z_o variation over frequency in a given application.
2	Low distortion and high slew rate: Minimize crossover distortion while optimizing voltage biasing.
3	Meet source or sink current requirements for driving large capacitive and resistive loads.
4	Capability to withstand high power dissipation and effectively manage thermal stress under worst-case conditions.
5	Maximizing output voltage swing headroom relative to programmable power supply voltage rails.
6	Inclusion of output overvoltage and overcurrent protection features: overload, short-circuit, and current limiting.

Line items 1 through 3 in Table 2-2 regarding the current booster are addressed in Sections 2 and 3 of the article.

2.1 Open-Loop Output Impedance

In this design, the current booster is configured as a complementary push-pull Darlington topology with unity gain buffering. Figure 2-1 and Figure 2-2 demonstrate that the open-loop output impedance is regulated by a small bias voltage applied to the bases of the transistors. Forward biasing the base-emitter junction of the NPN transistor (T1) allows the booster to source positive voltage and current, while forward biasing the PNP transistor (T2) enables it to sink negative voltage and current. The bias voltage directly affects the open-loop output impedance; higher bias levels result in lower output impedance, as shown in Equation 1.

Equation 1.

r_{o} \approx \frac{V_{A}}{I_{C}} \to Z_{C E} = \frac{r_{o p} \times r_{o n}}{r_{o p} + r_{o n}} \to Z_{C B O} = Z_{C E} ∥ R_{L}

Where,

r_o represents the BJT’s output impedance
V_A refers to the Early voltage
I_C denotes the bipolar collector current
r_op represents the NPN’s open-loop output impedance
r_on represents the PNP’s open-loop output impedance
Z_CE represents the parallel output impedance of the complementary Darlington pair
Z_CBO refers to the overall parallel open-loop output impedance

When the NPN transistor (T1) base-emitter junction is forward-biased, the current booster sources positive voltage and current at the output. Figure 2-1 illustrates the open-loop output impedance where the effects of Z_CE ∥R_L are shown to be less than 1Ω.

Conversely, when the emitter-base junction of the PNP transistor (T2) is forward-biased, the current booster sinks negative voltage and current at the output. Figure 2-2 demonstrates the open-loop output impedance, with the Z_CE ∥R_L effects yielding similar results. The combined open-loop output impedance remains consistent across the frequency range up to 1MHz.

Figure 2-1 Open-Loop Output Impedance (Z_CBO) with T1 Forward Biased

Figure 2-2 Open-Loop Output Impedance (Z_CBO) with T2 Forward Biased

The BJT transistor's forward biased voltage directly affects the open-loop output impedance; higher bias voltages lead to lower output impedance. The open-loop output impedance of the push-pull complementary BJT driver is primarily influenced by the output resistances (r_o) of the NPN (r_op) and PNP (r_on) transistors, as well as the load resistance (R_L). The open-loop output impedance of the Darlington current booster operates in parallel with R_L, as illustrated in Equation 1.