
Math
Functions: Logarithmic Circuits

Circuit Takes Square Root of Input Voltage:
11/20/97 EDNDesign Ideas 
Commutating Amp Performs Log Function: 02/16/95
EDNDesign Ideas 

Log amp uses capacitor charging law: 01/10/2002
EDN  Design Ideas / The novel logarithmic amplifier
in Figure 1 relies on the exponential charging
characteristics of a simple RC circuit. The
expression for the time, T, required for a
capacitor, C, to reach a voltage (VINVK) from 0V,
when charged through a resistor, R, with an applied
voltage of VIN, is VINVK=VIN (1eT/RC), where VK
is a fixed voltage. . 
Logarithmic Amplifier: Logarithmic amplifier
uses the log caracteristic of PN (diode, transistor)
to implement logarithmic function. The second
transistor is used as refference and with the PTK
resistor minimizes the dT/dV coeficient.

Model Fixed Point DSP Arithmetic in C: 03/18/99
EDNDesign Ideas / PDF contains multiple circuits 
scroll to find this circuit

Primer on Binary Arithmetic Rounding: 01/21/99
EDNDesign Ideas / PDF contains multiple circuits 
scroll to find this circuit

Reference stabilizes exponential current:
10/25/2001 EDN  Design Ideas / In an antilog
converter, the difference between the base voltages
of two transistors sets the ratio of their collector
currents: The use of matched transistors balances
the firstorder temperature coefficient but leaves a
temperaturedependent gain term, q/kT. Classic
antilog circuits use a thermistor in the drive
circuitry to correct this temperature dependency. . 
Software Filter Boosts Signal Measurement Stability,
Precision: 02/03/03 Electronic Design  Ideas
for Design / Small or embedded systems often require
a delicate measurement in the presence of high noise
or interference. Without fancy filtering hardware,
highspeed processing, or digital signal processing,
it can be difficult to extract a stable signal of...

Square Root Function Improves Thermostat:
09/30/99 EDNDesign Ideas / Perhaps the most
elementary rule of controlloop design theory is
that feedbackloop performance is fundamentally
linked to the careful choice—and stability—of loop
gain. Insufficient loop gain leads to poor setpoint
accuracy. Too much gain can induce feedback
instabilities, such as overshoot, ringing, and,
ultimately, oscillation. Therefore, the greater the
accuracy you expect from a control system, the more
critical maintaining nearoptimal loop gain becomes. 


