GB2303990A - Microphone amplifier - Google Patents

Abstract

A microphone amplifier comprises a transistor input stage having a bandwidth-reducing inductance 40 formed by a plurality of substantially equi-spaced wire turns 300 around a toroidal former 310. Any currents induced in the inductance tend to cancel out. Thus noise is reduced.

Description

MICROPHONE AMPLIFIER This invention relates to microphone amplifiers.

Microphone amplifiers for high quality audio processing systems have very stringent requirements on their performance. They must provide a high gain and sensitivity, to compensate for the low level of a microphone's electrical output signal, and yet despite their sensitivity to low input signals they should not be prone to induced noise from other sources.

Microphone amplifiers are commonly constructed as a two-stage circuit, with a high open-loop gain transistor input stage feeding a further operational amplifier stage. In effect, the operational amplifier stage forms part of a gain-limiting feedback loop around the transistor input stage.

However, it has been found that the overall bandwidth of the transistor input stage is often greater than that of the operational amplifier stage. Since the gain of the transistor input stage is limited by the feedback provided via the operational amplifier stage, this means that the transistor input stage gain is uncontrolled at frequencies outside the response of the operational amplifier stage, and so is prone to picking up and heavily amplifying induced noise at those frequencies. Accordingly, the imbalance in frequency response between the transistor input stage and the operational amplifier stage can provide a source of noise in the microphone amplifier circuit.

Standard techniques for reducing the bandwidth of an amplifier stage include adding either (i) resistance and capacitance, or (ii) inductance to the amplifier stage.

However, adding resistance to the transistor input stage of a microphone amplifier is undesirable, since any resistor is a source of noise. The noise introduced by such a resistor at the amplifier's input can badly affect the noise performance of the whole microphone amplifier.

Similarly, an inductor at the first stage of a high gain amplifier of this type can be very efficient at picking up magnetic interference from other parts of the circuit or from external sources. This induced noise would then be amplified by the remainder of the microphone amplifier (and any further amplifier stages connected downstream of this). Because of this problem, inductors have either been avoided in the input stages of microphone amplifiers, or attempts have been made to magnetically shield the inductors using expensive and complicated cases made of so-called mu-metal.

This invention provides a microphone amplifier comprising a transistor input stage having a bandwidth-reducing inductance formed by a plurality of substantially equi-spaced wire turns around a toroidal former.

The invention addresses the apparently conflicting problems of limiting the bandwidth of a transistor input stage in a microphone amplifier and avoiding magnetic pickup via a bandwidth-limiting inductance, by employing an inductance wound with equally spaced turns on a toroidal former. This symmetrical arrangement means that whatever the angle of incidence of a stray magnetic field with respect to the inductance, the field couples with equal lengths of winding in opposite directions.

Thus, any induced currents tend to cancel out.

Although the invention is advantageous when used with a single transistor input stage, preferably the amplifier has a further transistor input stage, the two transistor input stages acting in parallel to amplify complementary-polarity (positive and negative) balanced input signals, in which the bandwidth reducing inductances of the two input stages are mounted adjacent to one another. Here, because the second inductance is next to the first it tends to pick up equal noise components. However, because it is carrying an opposite polarity signal to the firstmentioned inductance, any residual induced noise components form a common mode signal which can be cancelled out when the two complementary polarity signals are subjected to further balanced signal processing.

Preferably, to improve the similarity of induced noise between the two inductances, the two inductances are mounted laterally adjacent to one another.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a schematic diagram of a microphone amplifier; Figure 2 is a schematic diagram of a toroidal inductor; and Figure 3 is a schematic diagram of a pair of adjacently mounted toroidal inductors.

Figure 1 is a schematic diagram of a microphone amplifier which receives balanced positive and negative input signals 10, 20 via dc blocking capacitors 30 to remove any phantom power or other dc signal present on the microphone leads. The signals are amplified by two respective symmetrical amplifiers, each based around a transistor input stage (a transistor 210) and an operational amplifier stage (an operational amplifier 220).

The microphone amplifier generates a balanced output comprising positive and negative output signals. The amplifier has a common mode gain which is very much lower than its differential mode gain.

The operational amplifier 220 and the transistor input stage 210 are connected via a common feedback loop, which acts to limit the gain of the transistor input stage 210. However, since the transistor input stage has a greater open loop bandwidth than the bandwidth of the operational amplifier feedback loop, inductors 40 are added into the transistor 210 emitter leads to limit the transistor stage bandwidth. The inductance of each inductor 40 is in fact 40 microHenries, and the construction and orientation of the inductors 40 will be described below with reference to Figures 2 and 3. Values of the remaining significant components are listed below in Table 1.

Figure 2 is a schematic diagram of a toroidal inductor comprising a plurality of equally spaced wire turns 300 on a toroidal ferrite former 310.

The spacing 320 of the tums is made as equal as possible, so that incident stray magnetic fields in any direction should couple with equal lengths of winding in opposite directions and so any induced currents should cancel out.

For the 40microHenry inductors used in Figure 1,10 turns of wire are evenly spaced around an 8mm outside diameter toroidal ferrite former.

Figure 3 is a schematic diagram showing a pair of adjacently mounted toroidal inductors 40', 40" on a printed circuit board 330. Here, the inductor 40' could be the inductor shown uppermost on Figure 1, and the inductor 40" could be the inductor shown at the lower half of Figure 1. The two inductors are mounted side-by-side on the printed circuit board 330, so that magnetic fields picked up by one inductor are substantially equal to those picked up by the other inductor. However, since one inductor carries the positive-going signal and the other carries the complementary negative-going signal to be amplified, the effects of induced noise form a common mode noise signal in opposite directions with respect to the positive and negative signals carried by the inductances, and so cancel out when the positive and negative signals are subjected to further processing as a balanced signal.

TABLE 1: MISCELLANEOUS COMPONENTS IN FIGURE 1 Resistors 100 100000 ohms 110 3000 ohms 120 100 ohms 130 22000 ohms 140 680 ohms 150 10000 ohms 160 4700 ohms 170 2400 ohms Capacitors 180 47 microfarads (electrolytic) 190 22 nanofarads 200 150 picofarads Semlconductors 210 LM394 transistor 220 NE5532 operational amplifier

Claims (4)

1. CLAIMS 1. A microphone amplifier comprising a transistor input stage having a bandwidth-reducing inductance formed by a plurality of substantially equi-spaced wire turns around a toroidal former.
2. 2. An amplifier according to claim 1, comprising a further transistor input stage, the two transistor input stages acting in parallel to amplify complementary-polarity balanced input signals, in which the bandwidth reducing inductances of the two input stages are mounted adjacent to one another.
3. 3. An amplifier according to claim 2, in which the two inductances are mounted laterally adjacent to one another.
4. 4. A microphone amplifier substantially as hereinbefore described with reference to the accompanying drawings.