SA2159

HANGZHOU SILAN MICROELECTRONICS CO.,LTD

Rev: 1.0

2005.04.22

Http: www.silan.com.cn

Page 5 of 11

Negative input currents

For negative input currents, the operation of Q1/D1 and Q3/D3 is symmetric with Q2/D2 and Q4/D4. pin 2 is

connected to ground, or very nearly ground potential (see the section below on symmetry adjustment for more

detail), Since the top pair versus the bottom pair transistors are inverted (NPN/PNP) and the bases are cross-

connected between the input (left) half and the output (right) half of each pair, the polarity (positive/negative, in

dB) of the gain is the same.

Symmetry adjustment

The layout design construction of the devices assures relatively good matching between the paired transistors,

but even small VBE mismatches can cause a dc output current to flow in pin 8, which will ultimately manifest itself

as a DC offset voltage. The DC offset voltage will be modulated by gain commands, and may become audible as

“thumps”if gain commands changes very large and fast.

Transistor matching also affects distortion. If the top half of the gain cell is perfectly matched, while the bottom

half is not well match, then the gain commanded by the voltage at pin 3 will affect the two halves of the core

differently. Since positive and negative halves of input signals are handled by separate parts of the core, this

gives rise to even order distortion production.

Figure 3. Typical THD versus symmetry voltage

Accordingly, the base of Q1 and Q4 are brought out to pin 2 and pin 4, respectively. This allows applying a

small static voltage differential to the two bases. The applied voltage differential must be set to the sum of the

Vbe mismatches around the core. Typical applications diagram includes a typical symmetry voltage application

circuitry. The symmetry trim pot controls primarily even-order harmonic distortion, and is adjusted for minimum

distortion in the output waveform.

Opposite polarity control

As may be seen from the equation, the bases of Q1 and Q4 can also be used as an additional control port,

which can be used as opposite control port of pin 3. Both pins must be applied to the control voltage

simultaneously, while the voltage applied between the two pins must be small difference. (A typical connection is

shown in Figure 4.) Either pin 3, or pins 2 and 4, or both ports together may be used for gain control.

Mathematically:Av = 10 - EC- / 0.118, or Gain = 20 log Av = - EC- / 0.0059, where the unit of Gain is dB.