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Logarithmic amplifier
| Details |
Inventors: Gilbert, Barrie;
Assignee: Analog Devices, Inc. (Norwood, MA)
Primary Examiner: Miller; Stanley D.
Assistant Examiner: Tran; Toan
Attorney, Agent or Firm: Wolf, Greenfield & Sacks
A multi-stage logarithmic converter of the "successive-detection" or "progressive-compression" type including circuitry providing an accurate, temperature-stabilized logarithmic transfer function. The gain stages are DC-coupled throughout, though each also employs a demodulator comprising a full-wave rectifier, allowing operation in both baseband and demodulating modes. The signal path is differential and is balanced, including the demodulators. Each gain stage is based on a differential amplifier, or "long-tail pair" operated in an open-loop mode and biased by a tail current generator which supplies a tail current that is both proportional to absolute temperature and compensated automatically for effects of finite transistor beta and base and emitter resistances. The demodulators are biased by a very low offset voltage which also is proportional to absolute temperature. The log intercept is temperature-stabilized by either employing a PTAT attenuator ahead of the complete amplification system or by introducing at the output node (a current summing junction) a current which varies with temperature in such a way as to offset intercept movement which otherwise would be generated. |
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DETAILED DESCRIPTION FIG.
7 depicts an ideal logarithmic response 40 for a device which generates an instantaneous voltage or current output proportional to the logarithm of its instantaneous input voltage.
A logarithm is determinate only for a dimensionless positive quantity, so the input voltage (or current) must effectively be divided by a "reference" voltage (or current).
The input voltage is represented by the variable V.
sub.
in and the reference voltage, by the variable V.
sub.
x.
The value of the logarithm is zero when the argument is unity and V.
sub.
in =V.
sub.
x.
The variable V.
sub.
x can be called the "log-intercept" or "intercept" voltage.
For a real logarithmic converter, of course, the output may not actually be zero for an input of V.
sub.
x, due to the effect of any input offset voltage, as well as limitations in the approximation function at low input levels.
The intercept voltage is not affected by these considerations and in practice is defined to high accuracy by extrapolation from the central, more-nearly ideal, region of the transfer function, corresponding to higher inputs.
According to the present invention, V.
sub.
x is constrained by design to be nominally 1 mV, and it is accurately trimmed during manufacture to that value.
The intercept voltages can be altered merely by subtracting or adding an offset to the output.
It is important to preserve a precise intercept, since ambiguity in the intercept translates directly to an uncertainty in the input magnitude.
Preservation of a precise intercept, therefore, is an important object of the converter design disclosed herein.
A voltage-input, current-output log converter must thus have an overall transfer function of the form ##EQU1## where the absolute value sign indicates that, because of the use of full-wave rectifiers, the output response for inputs of either polarity is identical.
Further, since the logarithm uses a base of 10, I.
sub.
Y may be viewed as a "scaling current" or as the "slope" (i.
e.
, current per decade).
The accuracy of the output is thus critically dependent on two variables, I
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