On Operational Amplifiers and Other Means
of Manipulating Voltage and Current
Short (50 min?) lecture: Design Expo stuff: poster, video etc. information
• I’ll have office hours and a review session
between the last day of class and the final.
– Exam review, likely Sunday late afternoon.
• HW6 will be assigned on April 3rd.
– Practice final—can’t assign until last topic talk is done.
Once you’ve done your topic talk…
• Send me your slides
– Pdf is fine, but I’d prefer power point (and both is
• Don’t believe I have the two from Tuesday yet.
• And yes, Merriam-Webster accepts that as an English word.
Analog—the bane of the
• In embedded systems, you often need to deal
with voltages and currents.
– Often the outputs you have don’t match the inputs
• Generally the current or voltage is too small
• Sometimes the voltage is too big.
• Sometimes the values are in the wrong range.
• Today we’ll touch on some ways of manipulating
– This is intended to give you some idea what options
are out there
• Often the details are tricky and/or annoying.
• We expect you may need to ask for help…
Examples of where you might have
• You have 5V for power, but some devices need 3.3V for
• You are using a device that generates too little current for
your ADC, you may want to amplify the current but hold
the voltage constant.
• You may be using a UART or other serial bus where one
device wants 3.3V or 1.7V and the other wants 5V.
• You may be driving a motor that needs [email protected] but you
can only drive [email protected], what do you do?
What are DC converters?
• DC converters convert one
DC voltage level to another.
– Very commonly on PCBs
• Often have USB or battery power
• But might need 1.8V, 3.3V, 5V, 12V and -12V all on the same board.
– On-PCB converters allow us to do that
Images from http://itpedia.nyu.edu/wiki/File:V_reg_7805.jpg, http://www.electronics-lab.com/blog/wp-content/uploads/2007/10/p1000255.JPG
• Probably the most common problem is
dropping power from 5V to 3.3V.
– Often because we’ve got a device that wants 3.3V
as Vcc and everything else wants 5V.
• The generic term for a device used to change
voltages is “DC converter”
– But when dropping it is sometimes called a
– Specific types are called “linear regulator”, “Low
Dropout (LDO)”, and “Switching regulator”
Different types of DC converters
• Simpler to design
• Low-noise output for noisesensitive applications
• Can only drop voltage
• Can be significantly more
complex to design
– And in fact must drop it by
some minimum amount
– The larger the voltage drop
the less power efficient the
• (All voltage dropped is
converted to heat).
– **Don’t use for project**
• Can drop voltage or
– “buck” and “boost”
• Generally very power
– 75% to 98% is normal
Linear regulators and capacitors
• Specification for regulators almost always include
– If you don’t have them, your output may get noisy and
cause all kinds of problems including reseting your chips.
• Too big is better than too small.
– May have required capacitor types (ceramic, etc.)
• Be sure to check (most linear regulators don’t…)
• We’ll briefly talk about using op-amps to do a
few basic things.
– Current buffering
– Threshold detection
• I’m not going to talk much about single supply
vs. dual supply.
Very idealized basics
• Can change the ground input to any voltage.
– Often just goes down to ground rather than –Vcc.
• How does this work?
And a bunch of others
Current to voltage and back.
Where would we have used a
– Used for the last 4 slides.
Serial bus issues
• There are devices that can convert data between voltage
– It uses two separate power supply rails, with the A ports supporting
operating voltages from 1.65 V to 3.6 V while it tracks the Vsupply, and
the B ports supporting operating voltages from 2.3 V to 5.5 V