866 Chapter 25

25.10.6.8 Transformerless Front Ends

Bringing the amplifier optimum source impedance
down to that of conventional dynamic microphones is
possible by means other than transformers. Reducing
the ratio of amplifier-inherent voltage and current
noises has this effect. Two main techniques, either alone
or in concert, are used:

1. Large-geometry devices have innately lower noise
   impedances. Even power-amplifier drivers have
   been used (e.g., 2N4918, BD538) but these tend to
   suffer from low transit frequencies (bandwidth) and
   beta (gain), which can lead to additional complexity
   in the circuitry.


2. Paralleling multiple identical input devices, and so
   proportionally increasing the noise current in rela-
   tion to the noise voltage, reduces the ratio between
   them (i.e., noise impedance).

PNP transistors, as mentioned elsewhere, have less
surface-recombinant noise / lower base-spreading resis-
tances than NPNs and are favored in this application.
The usual technique is to place two of these large
and/or multidevice input front-end amps—preferably
accurately matched—ahead of an electronic differential
amplifier, as shown in Fig 25-46. All the amplifier gain
is made within the first pair of stages, differentially
cross-coupled. This gain arrangement, rather than refer-
ring to ground, can afford reasonable common-mode
signal rejection. Differential input signals are amplified
since the reference for each of the two amplifiers is the
other amplifier, tied to an identical signal of opposite
polarity.

If the input signals to the two amps are identical in
phase and amplitude (common), the references for each
of the amplifiers are similarly waving up and down
sympathetically to the signal. There is no voltage differ-
ence for the individual amplifier to amplify; conse-
quently, there is no gain. For ordinary differential input
signals, the amplifiers operate conventionally, their
ground reference being a zero voltage point half-way
along the gain-determining variable resistor. This point
is a cancellation null between the opposite sense
polarity swings of the two amplifiers.
These amplifiers feed a conventional electronic
differential amplifier running usually at unity gain. In
order to maintain stage noise as low as possible, the
resistors are made as low in value as the devices can
sensibly stand. This arrangement is unmistakably a
bastardized instrumentation amplifier—a well-docu-
mented circuit configuration; the only thing of remark is
the pair of low-impedance optimized front-end stages.

A criticism (rightly) leveled at some implementa-

tions of this, including earlier-generation integrated

parts, is that the noise performance is significantly

worse at low amounts of gain than at high gains, where

Figure 25-46. Basic transformerless microphone-amplifier

arrangement.

Microphone

A. Microphone-amplifier arrangement.

B. Simple discrete input.

2SB737

BC169C

TL071

Gain

Antilog

BC169C

2SB737

Multi-input deviced amplifiers

optimized for low source impedance

Differential

amplifier

2N 4403 33 k 7

1 k 7

100 MF

100 MF

6 k 7

2 k 7

+48 V

VE

VE

+VE

+

+

+

6 k 7

Scn

470 pF

470 pF

1 k2

3

1

8

6

2

100 nF

100 nF

+VE

VE

470 pF

470 pF

1 k 7

100 MF

100 MH

+

100 MH

+

0

330 7

IN 4148 x4

Channel phantom control(ground active)

Chassis ground

Output

IC mic ampThat 1510

C. Integrated mic amp.

2.2 k (^7) 2.2 k 7
560 pF

= 2.2 k 7 Typical
values
2.2 k 7
560 pF
Non-polarized
10 MF
4.7 k 7 4.7 k 7
4.7 k 7
+VE
10 k 7
Non-
polar
220 MF
220 7
33 k 7
33 k 7
10 MF Non-polarized
3.9 k 7 3.9 k^7
10 MF30 k^7
68 pF
30 k 7
200 pF 10 k 7
10 MF10 k^7
VE
220 7
5534
50 k 7