Xref: utzoo rec.ham-radio:18088 sci.electronics:10351
Path: utzoo!utgpu!watserv1!watcgl!watnext!johnw
From: johnw@watnext.waterloo.edu (John Wieczorek)
Newsgroups: rec.ham-radio,sci.electronics
Subject: Re: How does one build LOG AMPS (>60dB dynamic range)
Message-ID: <13547@watcgl.waterloo.edu>
Date: 27 Feb 90 05:26:03 GMT
References: <596@massey.ac.nz>
Sender: daemon@watcgl.waterloo.edu
Reply-To: johnw@watnext.waterloo.edu (John Wieczorek)
Organization: University of Waterloo, Ontario, Canada
Lines: 28


	A fellow from New Zeland wanted to know how to go about 
assembling a log amp. Probabley the easiest way is to get an
Intersil ICL8048. I needs two resistors, a small 180pf cap and 
+/- 15 V and you're in business. It has 120 dB dynamic range for
current input and 60 dB for voltage input. 

	The voltage is just turned into current. The voltage precision is
then limited by the offsets of its internal op-amps. You can build
wider range circuits discretely with either chopper stabilized and 
/or low Ib op amps but the jump in cost and time from this chip in
performance is about 5x money and 30x time.

	For you information: log amps and anti-log amps exploit
the relationship that for a bipolar transistor Veb varies as the
log of Ic. The chip packages it all very nicely for you. Analog
Devices also has several entries in this field, but they are
hybrids vs Intersil monolithics which translates to higher cost.

	I've used the chip, it's cute.

						John Wieczorek


Disclaimer: The views and information contained herein are vicious lies,
and are in fact taken from the drunken musing of my three, beer swilling,
chain saw wielding but otherwise successfully house trained pet Blue Yukon
mongooses.