July 17, 1997

Simplicity pays off in circuit design

Mikhail Ioffe, Fishman Transducers

In circuit design, as in most aspects of life, simpler is often best. The availability of multiple op amps can seduce you into designing circuits with too many components.

In analog-circuit design, the theory that the simplest option is usually the best is not new. Designing circuits using fewer components is an old, often-forgotten art. In the past, every competent designer understood this precept, simply because it was almost the only way to work. Every extra tube or transistor would significantly raise the final product's cost. Today, unfortunately, designers often take a different tack. An abundance of literature and inexpensive parts on the market often lulls designers into thinking they have found a solution to their design problem, when, in fact, they have simply overcrowded their designs with parts. (Although these principles relate to analog design, you can also apply them to digital systems and software.)

The issue is not just saving a few cents in component costs. The overhead expense of using extra components can easily surpass the raw costs. You must reserve room on the board for the component. You must also buy, stock, handle, and install the component. More important, every extra part you put into your design inevitably reduces the end product's reliability and may add noise, distortion, and RF interference, as well as all sorts of dc, frequency, and thermal errors.

For example, designers commonly use a simple, low-battery-voltage detector in many battery-powered products (Figure 1a). IC₁ serves as a low-voltage detector in a standard configuration from the 8211's data sheet. R₁ provides threshold hysteresis, and C₂ makes the LED flash upon power-up. The flashing is a convenient feature that allows you to check the ba ttery's condition. (The longer the initial flash, the weaker the battery.) The 8211 is a good chip; it's accurate and consumes 280 µA of power. Nice, tidy circuit, right? Yes, but even such a simple structure has certain redundancies.

You can easily combine the R₃-to-R₄ divider, which establishes the 1.15V threshold at Pin 3, with the divider for V_CC/2. Moreover, capacitor C₁ does the job of C₂: It provides an initial flash. R₅ provides hysteresis to fight noise and V_CC fluctuations, but you can almost always do without it. The internal battery impedance, depending on the type, varies from single- to double-digit ohms; together with the 3- to 5-mA LED current, the impedance usually provides more than enough hysteresis. The circuit in Figure 1b incorporates these changes. Now, ask yourself: How often do you need 1% accuracy to indicate battery voltage? In most cases, 5% (over temperature) is adequate. If so, you can replace the 8211 (approximately $1 in quantity) with a couple of transistors in a simple configuration (Figure 1c). You can save at least $0.60. Just watch out for threshold drift, a function of the 2-mV/°C drift of Q₁'s V_BE.

Too many op amps

Consider another popular circuit, the notch filter. You can find Figure 2a in almost any textbook or op-amp cookbook. The circuit comprises an inverter and a state-variable filter. It's easy-to-design and predictable, and you can easily tune it for a desired attenuation or boost (in bandpass operation) and for the Q you need. What more could you want? You should always wish for lower current consumption, for example. The circuit in Figure 2b works well with 2.5 times lower current drain and with fewer components. This configuration is an inverter plus a bandpass filter. You can find the design of the bandpass filter in any filter cookbook.

You can change the amount of attenuation by adjusting the resistor values, and you can set Q by varying the ratio of C₁ to C₂. You must simply keep the product of the capacitor values constant to avoid changing the center frequency. The depth of the notch depends not only on the value of R₃ (with given values of R₁ and R₂), but also on the filter-stage gain. This gain is a function of the tolerance of the dual potentiometer, P₁ (20% for most types). You should define the minimum allowable notch depth and then choose R₃ for the worst-case scenario for the potentiometer mismatch (P_1A 20% high, P_1B 20% low).

Figure 3 shows the response of the circuit for two distributions of potentiometer values. Despite the considerable differences in response, the performance may be acceptable in some applications. Besides, you can easily procure potentiometers that track better than the barbaric ±20% cited. The Q of the circuit also depends on the potentiometer tracking but in a good way: Q gets higher only with deeper notches. This type of filter works well in audio applications for notching unwanted frequencies, thus fighting acoustic feedback.

The circuit in Figure 4a is a woofer power amplifier for a biamp "combo" amplifier used in professional music applications. Combo amplifiers combine the amplifier and loudspeaker in one enclosure. These amplifiers are virtually ubiquitous, but the one shown specifically targets acoustic-guitar amplification. The term "biamp" denotes an amplifier with two separate amplifiers for the low- and high-frequency bands and an electronic crossover. An electronic crossover is much cheaper and easier to design than a conventional passive crossover. The first stage provides "room compensation," which, in fact, is just low-frequency (bass) boost. The second stage is a 60-Hz, second-order highpass filter that protects the woofer from large infrasonic-level fluctuations, which are common with pressure-type piezoelectric transducers used in acoustic amplification.

The third stage is a 2.4-kHz lowpass filter, and the fourth stage is the power amplifier itself. Although the circuit in Figure 4a looks straightforward, you can simplify it (Figure 4b). You can easily obtain the few decibels of low-frequency boost in the power-amplifier stage. You can also combine the 60-Hz lowpass and 2.4-kHz highpass filters into one stage, thanks to the relatively large difference in frequencies. Only a small difference exists between the frequency responses of circuits using separate and combined filter stages. With available filter-design software, you can easily figure out and minimize this difference if necessary.

You don't need a separate highpass-filter stage for the high-frequency-tweeter power amplifier (Figures 5a and 5b). Filtering can occur in the power amplifier itself (which you can consider as an operational amplifier). However, don't try to use the same trick for the low-frequency band. Doing so may cost you the price of the output devices, because the power amplifier can oscillate at a very high frequency. (Remember the Nyquist theory and Bode plots?)

Eschew op-amp mania

A proposed solenoid-driver circuit (Figure 6a) applies 12V for 20 msec to the solenoid and then reduces the voltage to 7V to save power (Reference 1). The circuit uses a differential amplifier, IC_1B; a precision rectifier, IC_1C; a high-input-impedance follower, IC_1D; a power amplifier, IC₂; and three power supplies: ±15 and 16V unregulated.

You could build this type of circuit 30 years ago with one Darlington transistor, a 3W resistor, and one capacitor; you could build it 20 years ago with two Darlingtons and one much smaller capacitor (Figure 6b). If you use Darlingtons with gain higher than 10,000, you can assume that the time constant (how fast V_A drops from 5V to approximately 1V of V_BE) of the circuit is a function of C₁ and R₅. For lowercase tau=20 msec and R₅=1 kilohms, you need C₁=20 nF.

The following ground rules may prove helpful in gaining simplicity, efficiency, and elegance in circuit designs:

• Beef up your split supply. It's common to use a voltage divider to establish V_CC/2 in single-supply portable designs. That's OK, but when the number of signal-processing stages grows (say, to more than four) and if every stage is ac-coupled with the previous stage, then it's beneficial to create a robust V_CC/2 with a buffer. Thus, you eliminate a lot of electrolytic capacitors. These capacitor are among the least reliable components, so the fewer you have in your design the better.

• Combine voltage dividers. If you need several voltage-reference points, try to combine them in one divider or buffer stage, as with the V_CC/2 example.

• Using op amps, combine two or more features into one stage. You can always obtain filtering with gain, often with a high input impedance. The same situation is true with tone controls, phase inverters, balanced line driv-ers, receivers, and other such features. Sometimes, you can combine features of two different-function circuits, such as a low-battery indicator and a clipping-condition signal, into one.

• Avoid using extra op amps just to exploit some of their strong features. For instance, don't automatically buffer a 1-kilohms level-control potentiometer connected to the output of an op amp. It's more than possible that the 250 ohms or so maximum output impedance of the amplifier is low enough. If you really need zero loading, consider putting the level control in the feedback loop of an inverter. Also, don't use op amps just because they're cheap or because you have an extra one in a package. Your signal is unlikely to benefit from an extra element in the chain.

• Try to cut bells and whistles at the conceptual stage of a design. At first, certain features may seem useful and convenient (or good for marketing), but they often turn into a nightmare when producing, testing, or using a product. An example is a device with a flashing-LED low-battery indicator. This feature is nice if you can tolerate the clicks in the signal chain that arise from the current pulses. As a rule, extra features drive up the product's cost and drive down its reliability.

• Avoid using fancy, hard-to-find, or expensive parts. For example, transistors can often substitute for an op amp, and a simple zener diode can often replace an expensive voltage regulator.

• After you finish a design, take a last look and get rid of all unnecessary 9.1- and 11-kilohms resistors, for example. Recalculate some time constants so you can also reduce capacitor values. Your company buyers and stockroom people will be grateful .

| EDN Access | Feedback | Table of Contents |