So Many Amplifiers to Choose From: Matching Amplifiers to Applications
By Ron Mancini (Email: rmancini@ti.com)
Staff Scientist, Advanced Analog Products
Published in Analog Applications Journal, 3Q 2005
Introduction
Amplifier selection can be confusing due to the variety of amplifier types available, many of which perform similar functions. Amplifier types include op amps, instrumentation amps, audio amps, differential amps, current feedback amps, high-frequency amps, buffers, and power amps. Amplifier names often reflect their initial applications rather than their current use. For instance, "Op amp" is short for operational amplifier, originally used for mathematical functions in analog computers.
All semiconductor amplifiers discussed here rely on internal or external feedback. Transistors, the fundamental building blocks of amplifiers, have characteristics that vary with manufacturing tolerances, temperature, stress, and time. Without feedback, amplifier gain can be uncontrollable, varying by a factor of 10 or more. Feedback is crucial for controlling gain variations, but it can introduce issues like overshoot, ringing, and oscillation. Internal compensation addresses the tendency to overshoot or oscillate, making the amplifier stable under recommended conditions. However, any amplifier with a gain greater than one can oscillate under certain conditions. Amplifiers requiring external feedback need external compensation components to prevent oscillation or saturation. It is essential to investigate the compensation strategy when selecting an amplifier to avoid building an oscillator instead of an amplifier.
Operational Amplifiers (Op Amps)
Operational amplifiers (op amps) are versatile and, within their limitations, can substitute for most other amplifier types. Effective design involves understanding the op amp's limits and knowing when to use alternative solutions. Often, external components, rather than the op amp itself, impose these limits. Figure 1 illustrates a generalized op amp circuit, and Table 1 shows how external component choices affect circuit performance. Replacing impedances with capacitors creates frequency-dependent functions, and the placement of the input signal alters the transfer function. Many specialty amplifiers exist to overcome op amp limitations, but the distinction has blurred, often allowing a choice between an op amp or a specialty amplifier for a given task. This discussion focuses on specialty amplifiers but references the op amp as the baseline, as specialty designs often arise from op amp deficiencies.
Figure 1. This op amp configuration produces many different circuits
A generalized op-amp circuit diagram showing an operational amplifier with inputs labeled 'V-' and 'V+', and outputs labeled 'VOUT'. It has feedback components ZG and ZF, and input components Z1 and Z2. The diagram illustrates how changing these components creates different circuit functions.
Table 1. Changing component values yields many op amp circuits
| CIRCUIT TYPE | V₁ | V₂ | ZG | ZF | Z₁ | Z₂ |
| Inverting amp | Input signal | Ground | Determined by gain | Determined by gain | Open | ZG || ZF |
| Noninverting amp | Ground | Input signal | Determined by gain | Determined by gain | Open | Open |
| Inverting integrator | Input signal | Ground | RG | CF | Open | ZG || ZF |
| Buffer | Ground | Input signal | Open | Short | Open | Open |
| Subtractor | Input signal - | Input signal + | RG | RF | RG | RF |
Buffer Amplifiers
This discussion focuses on voltage buffer amplifiers, as current buffer amps are typically used within feedback loops. Voltage buffers provide a gain of one, exceptionally high input impedance, and very low output impedance. In an op amp, the input voltage encounters a load impedance comprising input components and the op amp's input impedance. In a buffer circuit, the load impedance is solely due to the op amp. This is a primary limitation: external components always load the input signal, which is detrimental to performance when the signal source has significant resistance. Buffer amplifiers or instrumentation amplifiers are key to solving input impedance issues.
Op amps exhibit output impedance characteristics similar to other amplifiers, but feedback modifies the op amp's output impedance. The output stage's resistance is a primary component. An emitter-follower configuration typically has low inherent output impedance (around 25 Ω). This impedance increases with frequency, introducing poles and errors at high frequencies. Rail-to-rail op amps often use a common-collector output stage, where impedance depends on the load and can be high (kilohms). Circuit loop gain significantly lowers the overall output impedance. At low frequencies, op amps generally have very low output impedances (fractional ohms). However, output impedance rises with frequency as the op amp's gain decreases. High output impedance can cause DC errors due to load currents and stability issues with output capacitors creating poles. For high load currents, selecting an op amp designed for the specific load is recommended. Buffers offer an advantage over standard op amps for low output impedance because their loop gain is always maximum, and their output stages are designed for low impedance.
Some op amps become unstable with capacitive loads, while others handle them well. Op amps designed for large capacitive loads have low-resistance output stages but may sacrifice speed due to larger output transistors. In summary, output impedance might necessitate using a buffer, a specialized op amp, or a power amp.
Figure 2. Either of these symbols represents a buffer
Two symbols representing a buffer amplifier. Both symbols show an input terminal and an output terminal, with a gain of +1. One symbol is a triangle with a '+' and '-' input and an output. The other is a simplified block with an input and output arrow.
Subtractor or Difference Amplifiers
Building an op amp circuit requires external resistors and capacitors. Table 1 shows that when all external impedances are resistive and equal, the circuit functions as a subtractor. Equation 1 provides the general subtractor equation:
VOUT = V2 · (RF + RG) / RG - V1 · RF / RG (Equation 1)
When R2 = RF and R1 = RG, Equation 1 simplifies to Equation 2:
VOUT = (V2 - V1) · RF / RG (Equation 2)
For common-mode voltage rejection (where V2 = V2Signal + VCM and V1 = V1Signal + VCM), the conditions for Equation 2 require excellent resistor matching. Designers implement Equation 2 using op amps with discrete resistors or integrated circuits (ICs). The op amp approach is more flexible, allowing for ICs with multiple op amps and easy adjustment of discrete resistor values for gain changes. However, discrete resistor matching can be poor; 1% tolerance resistors might yield only 24.17 dB of common-mode rejection, whereas IC subtractors achieve over 100 dB by tuning front-end transistors and using on-chip film resistors, often laser-trimmed for accuracy.
Subtractors using resistors do not present the highest possible impedance to the load. Measurements like strain-gage bridge measurements require high common-mode rejection to eliminate noise, but bridge circuits have output resistance that interacts with the subtractor's input resistance. These applications need high-input-impedance circuits that eliminate common-mode noise.
Instrumentation Amplifiers (IAs)
Instrumentation amplifiers (IAs) offer high input impedance and high common-mode rejection, making them ideal for many instrumentation and industrial applications (see Figure 3). In the three-op-amp IA, each input is connected to the noninverting input of an op amp, providing the highest input impedance without complex feedback. The subtractor stage (R3-R6 and A3) provides common-mode rejection, while op amps A1 and A2 buffer the subtractor, maintaining high input impedance. This circuit effectively amplifies bridge signals and rejects common-mode voltage with minimal error. A key advantage is that a single, nonmatched resistor sets the gain, eliminating resistor matching issues. Downsides include increased cost, signal delay, and a reduced common-mode voltage range.
The two-op-amp IA also uses a subtractor for high common-mode rejection (Figure 4) and features two noninverting op amp inputs, ensuring high input impedance. It offers a wider common-mode voltage range than the three-op-amp version due to fewer stacked op amps. However, it suffers from unequal stage delays for input signals: the inverting input has two delays, while the noninverting input has one. These unequal delays cause distortion at frequencies above DC, increasing with signal frequency, though it is minimal within the IA's typical operating range.
Figure 3. This instrumentation amplifier uses three op amps
A three-op-amp instrumentation amplifier circuit. It shows two input buffers (A1, A2) connected to a difference amplifier (A3). The gain is determined by resistors RG, R1, R2, R3, R4, R5, R6, and a reference voltage VREF.
Figure 4. This instrumentation amplifier does the job with fewer parts
A two-op-amp instrumentation amplifier circuit. It uses two op-amps (A1, A2) and resistors RG, R1, R2, R3, R4. It's presented as a simpler alternative to the three-op-amp version.
Current Feedback Amplifiers (CFAs)
Current feedback amplifiers (CFAs) are designed for high bandwidth. Previous amplifiers discussed were Voltage Feedback Amplifiers (VFAs), whose open-loop gain decreases significantly at low frequencies (often starting at 10 Hz) with a -20 dB/decade slope. This gain reduction leads to poor accuracy at high frequencies. Figure 5 illustrates that VFAs lose gain at high frequencies, while CFAs maintain high gain well into very high frequencies. VFAs operate in a frequency spectrum where gain is decreasing, which aids stability, as circuits with gain less than one are unconditionally stable. CFAs, however, maintain their gain as frequency increases, lacking this "hidden" stability advantage. Circuits stable with VFAs may become unstable with CFAs. Additionally, CFA input leads and feedback resistors are sensitive to stray capacitance, requiring careful layout considerations. Vendor evaluation boards are recommended for testing and layout examples.
Figure 5. Frequency responses of VFAs and CFAs
Two graphs showing frequency responses of Voltage Feedback Amplifiers (VFAs) and Current Feedback Amplifiers (CFAs). The Y-axis is 20Log(Gain) and the X-axis is Frequency (Log scale). The VFA shows decreasing gain at higher frequencies, while the CFA shows relatively constant gain.
Figure 6. Simplified model of a CFA
A simplified model of a Current Feedback Amplifier (CFA). It shows an inverting input and a non-inverting input. The circuit involves components like GB, ZB, Z(I), GOUT, ZOUT, and a feedback resistor (implied by ZB).
Figure 7. Notice the special decoupling used in a WFGA application
A circuit diagram for a Wideband Fixed-Gain Amplifier (WFGA) application, showing specific decoupling components (capacitors, ferrite beads) and resistors for a THS4303 amplifier. It also includes a graph of the "Small Signal Frequency Response" for this circuit.
High-Frequency Amplifiers
High-frequency amplifiers are typically fixed-gain amplifiers. Those operating in the gigahertz range are often hybrid amplifiers, constructed from discrete transistors and passive components on a substrate. This construction is costly but necessary for achieving high performance in demanding applications. Wideband Fixed-Gain Amplifiers (WFGAs) are a newer IC product utilizing special high-frequency processes and non-standard internal compensation techniques to achieve high Gain-Bandwidth Product (GBW), often up to 10 GHz. These amplifiers are available in fixed configurations with fixed gains. Any high-frequency amplifier requires careful handling, and the cautions for CFAs apply, along with strict adherence to datasheet applications information.
Fully Differential Amplifiers (FDAs)
Fully Differential Amplifiers (FDAs) create and utilize fully differential signals. Many Analog-to-Digital Converters (ADCs) require differential input signals for optimal performance, and FDAs can easily convert single-ended signals to differential signals. Previously, this conversion required two op amps and matched resistors, a complex and costly design. FDAs simplify this process and offer advantages like fewer components (Figure 9). Beyond simplicity and cost, FDAs provide a common ground point determined by the ADC. Differential signal transmission ensures that coupled noise is common-mode noise, which the ADC or receiver can reject using its common-mode rejection capability.
Figure 8. Traditional single-ended to differential converter circuit
A traditional circuit for converting a single-ended signal to a differential signal using two op-amps and several resistors. It shows input VIN, reference voltages VREF+, VREF-, and outputs VOUT+, VOUT-.
Figure 9. The FDA converts a single-ended signal to differential
A circuit diagram showing how a Fully Differential Amplifier (FDA), like the THS41xx, converts a single-ended signal to a differential signal. It shows input VIN, reference VOCM, and outputs VOUT+, VOUT-.
Power Amplifiers (PAs)
When an op amp needs to deliver more than a few hundred milliamps at several volts, a Power Amplifier (PA) is required. PAs are linear amplifiers capable of handling significant power, not switching-type amplifiers. Effective power amplifier design involves more than just heat sinks; critical functions include current sense, overload shutdown, and paralleling capabilities (Figure 10). The OPA569, for example, handles considerable current for a precision op amp and includes features like current limit set, current monitor, parallel connections, enable, current limit flag, and thermal limit flag. While other PAs handle higher currents and voltages, these features are common among modern devices.
Figure 10. The new breed of power amplifier does more than handle power
A block diagram of a power amplifier (OPA569), highlighting its features like Current Limit Set, Current Limit Flag, Thermal Flag, Enable, and options for parallel outputs.
Audio Amplifiers
Audio amplifiers have been a distinct category since the 1950s due to their high volume. Most semiconductor manufacturers offer a range of audio amplifiers, from simple op amps to complex switching power amps.
The Next Step
This guide introduces specialty amplifier areas. To proceed, consult online resources. Reputable IC manufacturers provide websites with extensive information on their products and applications. Exploring these sites can help identify suitable ICs for problem-solving, but do not overlook the applications information, which is crucial for efficient design. Some manufacturers provide abundant application data, while others offer little. This information significantly impacts the speed and completeness of your work. Understanding concepts like decoupling capacitors or thermal runaway through application notes can prevent costly hardware mistakes.
References
- Ron Mancini, Op Amps for Everyone (Newnes Publishers, 2003). An earlier 2002 edition is available at www-s.ti.com/sc/techlit/slod006
- Ron Mancini, "Worst-case circuit design includes component tolerances," EDN (April 15, 2004), pp. 61–64. Also available online at www.edn.com/article/CA408380.html
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