1 Audio Amplifiers 2 Buffer Amplifiers/Driving Cap Loads 3 ...

1 SIGNAL Amplifiers H Op Amp History1 Op Amp Basics2 Specialty Amplifiers3 Using Op Amps with Data Converters4 Sensor Signal Conditioning5 Analog Filters 6 Signal Amplifiers1 Audio Amplifiers2 Buffer Amplifiers /Driving Cap Loads3 Video Amplifiers4 Communications Amplifiers5 Amplifier Ideas6 Composite Amplifiers7 Hardware and Housekeeping Techniques OP AMP APPLICATIONS SIGNAL Amplifiers Audio Amplifiers CHAPTER 6: SIGNAL Amplifiers Walt Jung, Walt Kester SECTION 6-1: Audio Amplifiers Walt Jung Audio Preamplifiers Audio signal preamplifiers (preamps) represent the low-level end of the dynamic range of practical Audio circuits using modern IC devices.

2 In general, amplifying stages with input signal levels of 10mV or less fall into the preamp category. This section discusses some basic types of Audio preamps, which are: Microphone including preamps for dynamic, electret and phantom powered microphones, using transformer input circuits, operating from dual and single supplies. Phonograph including preamps for moving magnet and moving coil phono cartridges in various topologies, with detailed response analysis and discussion. In general, when working signals drop to a level of 1mV, the input noise generated by the first system amplifying stage becomes critical for wide dynamic range and good signal-to-noise ratio. For example, if internally generated noise of an input stage is 1 V and the input signal voltage 1mV, the best signal-to-noise ratio possible is just 60dB.

3 In a given application, both the input voltage level and impedance of a source are usually fixed. Thus, for best signal-to-noise ratio, the input noise generated by the first amplifying stage must be minimized when operated from the intended source. This factor has definite implications to the preamp designer, as a "low noise" circuit for low impedances is quite different from one with low noise operating from a high impedance. Successfully minimizing the input noise of an amplifier requires a full understanding of all the various factors which contribute to total noise. This includes the amplifier itself as well as the external circuit in which it is used, in fact the total circuit environment must be considered, both to minimize noise and maximize dynamic range and signal fidelity.

4 A further design complication is the fact that not only is a basic gain or signal scaling function to be accomplished, but signal frequency response may also need to be altered in a predictable manner. Microphone preamps are an example of wideband, flat frequency response, low noise Amplifiers . In contrast to this, phonograph preamp circuits not only scale the signal, they also impart a specific frequency response characteristic to it. A major part of the design for the RIAA phono preamps of this section is a systematic analysis process, which can be used to predictably select components for optimum performance in frequency response terms. This leads to very precise functioning, and excellent correlation between a computer based design and measured lab operation.

5 OP AMP APPLICATIONS Microphone Preamplifiers The microphone preamplifier (mic preamp) is a basic low level Audio amplification requirement. Mic preamps can assume a variety of forms, considering the wide range of possible signal levels, the microphone types, and their impedances. These factors influence the optimum circuit for a specific application. In this section mic preamps are discussed which work with both high and low impedance microphones, both with and without phantom power, and with transformer input stages. Single-Ended, Single-Supply high -Impedance Mic Preamp A very simple form of mic preamp is shown in Figure 6-1. This is a non-inverting stage with a single-ended input, most useful with high -impedance microphones such as dynamic and piezoelectric types.

6 As shown it has adjustable gain of 20-40 dB via RGAIN, and is useful with Audio sources with 600 or greater source impedances. Figure 6-1: A single-ended,single-supply mic preamp The U1 op amp can greatly affect the overall performance, both in general amplification terms but also in suitability for single supply operation (as shown here). In terms of noise performance, the U1 device should have a low input noise with 500 sources, with the external circuit values adjusted so that the source impedance (microphone) dominates the overall source resistance. For very low noise on 5V supplies, very few devices are suitable. Among these the dual SSM2135 or the OP213, and AD822/AD823 stand out, and are recommended as first choices.

7 For very low power, minimal quiescient current parts like the AD8541 can be considered. Many other low noise devices can also work well in this circuit for total supply voltages of 10V or more, for example the OP275, and OP270/470 types. The circuit is also easily adapted for dual supply use, as noted below. SIGNAL Amplifiers Audio Amplifiers In this circuit, gain-determining resistors R1||R2 (where R2a + R2b = RGAIN) are scaled such that their total resistance is less than the expected source impedance, that is 1k or less. This minimizes the contribution of the gain resistors to input noise, at high gain. As noted, gain of the circuit is adjusted in the feedback path via resistor RGAIN. In a system sense, control of a microphone or other low level channel signal level is preferably done after it has undergone some gain, as the case here.

8 RGAIN can of course be a fixed value. Because of the single supply operation, input/output coupling is via polar capacitors, namely C1, C2, and C3. C4 is a noise filter, and C5 a bypass. For lowest noise in the circuit, the amplifier biasing must also be noiseless, that is free from noise added directly or indirectly by the biasing (see Reference 1). Resistors with DC across them should have low excess noise (film types), or be AC-bypassed. Thus R1, R2, R3, R4, R7, and R8 are preferably metal films, with R7-R8 bypassed. A bias provided from R7-R8 biases the output of U1 to near mid-supply. If higher supply voltage is used, R7-R8 can be adjusted for maximum output with a particular amplifier.

9 For example, with low bias current, rail-rail output op amps, R7 and R8 should be high , equal values ( 100k ). While the OP213 or SSM2135 for U1 is optimum when operating from lower impedance sources, FET input types such as the AD82x families (or a select CMOS part) is preferable for high impedance sources, such as crystal or ceramic mics. To adapt the circuit for this, R3 and R4 should be 1M or more, and C1 a F film capacitor. Bandwidth using the OP213 or SSM2135 is about 30kHz at maximum gain, or about 20kHz for similar conditions with the AD822 (or AD820). Distortion and noise performance will reflect the U1 device and source impedance. With a shorted input, an SSM2135 measures output noise of about 110 Vrms at a gain of 100, with a 1kHz THD+N of at 1 Vrms into a 2k load.

10 The AD820 measures about 200 Vrms with THD+N for similar conditions. For both, the figures improve at lower gains. The circuit of Figure 6-1 is a good one if modest performance and simplicity are required, but requires attention to details. The input cable to the microphone must be shielded, and no longer than required. Similar comments apply to a cable for RGAIN (if remote). To adapt this circuit for dual supply use, R3 is returned to ground as noted, plus the bias network of R7, R8 and C4 is eliminated. U1 is operated on symmetric supplies ( 5V, 15V, etc.), with the VS rail bypassed similar to +VS. Coupling caps C1, C2 and C3 are retained, but must be polarized to matched the amplifier used (or non-polar types).