Is there a clever way to make a current amplifier which acts like a NPN transistor except that it has 0 Vbe? That is, with a "base" resistor of 10k and an input voltage of 100mV, 10uA gets amplified into 1mA at the "collector"? Preferably a method that does not use an IC's (opamps, etc..)
The OPA861 comes to mind but it's an 8 pin IC.. Clipped from datasheet... " The OTA or voltage-controlled current source can be viewed as an ideal transistor. Like a transistor, it has three terminals?a high impedance input (base), a low-impedance input/output (emitter), and the current output (collector). The OPA861, however, is self-biased and bipolar. " D from BC
A cascade of an NPN and PNP emitter followers have approximately zero offset voltage.
Integrates amplifiers made of two complementary emitter followers in cascade make very fast, fairly low offset voltage current amplifiers. I know you asked for a non integrated solution, but you might be able to get some ideas how this is done from this data sheet.
Something roughly like...
+5 | | 10k | | | | c +----------b npn | e | | e 50r----+-----b pnp | | c gnd 10k | | | gnd gnd
John
Smoke and mirrors make everything work ;-)
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |
_Magic_ smoke and current mirrors.
Cheers,
Phil Hobbs
_Magic_ is _MY_ domain ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Jim Thompson a écrit :
But mirrors is sure not :-)
-- Thanks, Fred.
As opposed to the real world.
-- Bill Sloman, Nijmegen
I like the opa861idbv with its cute SOT23-6 package.
TI also offers the Burr-Brown SO-8 package opa860, which has the same OTA stage, but adds an output buffer.
These parts replace the opa660, which was discontinued. Read the opa660 datasheet to see the schematic.
While TI calls these operational transconductance amplifiers (OTAs), and they characterize them as ideal active transistors, they're actually basically current-mode opamps with the transconductance node exposed and the Gv=1 output stage either omitted or made available with a separate connection pin, as in the opa860 (and opa660). This type of OTA functionality can be had in several Analog Devices opamps, like the ad846 and ad844, which bring out the transconductance pin, labeling it "compensation".
TI's ability to set the quiescent current of the input transistors is a nice touch, because you can reduce the +input current (what they call the base) and reduce the output-current offset, at the expense of speed and a reduced the maximum output current capability (note, the maximum output current can substantially exceed the quiescent current).
Hell, you wouldn't know real Magic if it jumped up and bit you in the ass.
Good Luck! Rich
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