Cascading Precision Op Amp Stages for Optimal AC and DC Performance

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1 Introduction

Operational amplifiers (op amps) employed in various applications may be called upon to amplify very small signals, requiring them to operate with high closed-loop gains in the hundreds, or thousands of volts per volt (V/V). If the required gain is in the range of tens to a few hundred V/V, a single precision op amp can provide the desired electrical performance. However, once the gain requirements become significantly higher, a single-stage approach mght not provide adequate levels of performance.

2 Voltage Offset

Voltage offset occurs due to slight mismatches between transistors that make up the differential input pair of an op amp. On some products, TI employs unit-level eTrim or laser trimming during production test to reduce this offset as much as possible. However, it is not practical to remove the voltage offset entirely, so the engineer must account for it in the design process.

Figure 2-1 shows how the offset voltage can be modeled as a DC voltage generator Vosi, in series with the non-inverting input of an op amp. The exact value of Vosi will vary between components and can have either positive or negative polarity. Typical and maximum Vosi values are specified in the op amp’s datasheet Electrical Characteristics table.

Figure 2-1 Amplifier stage with voltage offset model

There is always a DC error voltage at the output of an amplifier circuit. This output error is mainly caused by the Vosi multiplied by the non-inverting gain of the amplifier circuit, which appears on the output as Voso. A higher gain circuit will produce a greater Voso at the output.

There are additional factors that add to the total DC output error; finite common-mode rejection (CMRR), power supply rejection (PSRR), open-loop gain (Aol), and input bias currents (Ib) flowing through the source resistances. Although they must be considered in every design, in the interest of brevity, these errors will not be included in this discussion.

3 Bandwidth

Typical voltage feedback op amps have a low frequency pole present in their AC response, limiting the bandwidth of the device. The frequency response is similar to that of a low pass filter, with reduced gain at higher and higher frequencies. The bandwidth of the amplifier circuit is determined by dividing the gain bandwidth product (GBW) of the op amp by the closed-loop gain of the amplifier circuit. The GBW is specified in the op amp’s datasheet Electrical Characteristics table.

Equation 1.

For example, the GBW of the OPA2210, a precision op amp, is 18MHz. For an application requiring a high gain of 1000 V/V (60dB), the effective bandwidth of a single-stage amplifier is a mere 18kHz (AC response of this circuit is shown in Figure 4-2). While this may be plenty for a near-DC application, there are many applications that demand both high gain and high bandwidth. A high speed op amp with a higher GBW product can be used to extend the bandwidth of the system at high gains, but typically it will come at the expense of DC precision.

Few devices in the marketplace can satisfy the needs of high DC precision, gain and bandwidth in a single amplifier stage. Instead, multiple precision op amps can be cascaded to preserve bandwidth while maintaining low offset and low noise for high gain applications.