Suppose we have an opamp connected in the inverting amplifier configuration:

 in        R1                      R2       
 o----+--/\/\/\------------+-----/\/\/\-------------+----o out
                           |                        |
                           +--|-\                   |
                              |  \__________________|
             R_BIAS           |  /
      +------/\/\/\-----------|+/ OPAMP
      |
     GND


The gain of this thing is just -R2/R1, i.e., if R2 is 10000 ohms and R1 is 5000 ohms, then the output will be twice as large as the input, and with its sign flipped. That is well and good, but we are concerned about the noise introduced to the circuit. Noise is introduced both by the opamp itself, which, in our abstract view, is intrinsic to the amplifier; and by Johnson noise in the two resistors in the feedback network.

Horowitz and Hill give an expression for the noise of this circuit on page 447 of Art of Electronics, but they dedicate remarkably little space to it. Only about 8 3/4 square inches, actually, and their answer is somewhat confusing.

First, the amplifier noise. The amplifier noise is specified as an equivalent voltage noise e_a and current noise i_a seen at one (or both, you never can be sure what the datasheet is quoting) of the opamp terminals. We can pretend that the voltage noise e_a is all clumped at the noninverting input to the opamp. We then multiply that by the "noise gain," which is the transfer function from the opamp's noninverting input to the output of the circuit: 1+R2/R1. Done.
Last night I started working on the x0xb0x, the Roland TB-303 Sequencer/Synthesizer clone that I am building from a kit. I remembered soldering as being tedious and stressful, but now I find it a relaxing and satisfying diversion. Reminds me of when Daniel at LHO told me, "Anyone can feel good about themselves when soldering!" Unlike so many other activities at school, you make guaranteed progress towards a goal, and it doesn't require much thinking. Anyway, I finished building and testing the power supply and am ready to move on to the voltage controlled oscillator.

In the last 24 hours:
  • Assembled and tested x0xb0x power supply, in the Secret Underground Laboratory
  • Slept a lot
  • Made Norwegian waffles for breakfast
  • Received a shipment of books from Duffy Littlejohn
  • Scoped out the Rochester and Southern rail yard
  • Fraternized with [livejournal.com profile] vyncentvega
  • Looked at my TA evaluations. They were very positive except the students say I grade homework too slowly.
  • Bought stock in the company that owns Rochester and Southern, just for the hell of it
  • Ate some more waffles
  • Rented a truck from home depot to move some more gravel to finish our hot tub foundation
  • Got this truck stuck in mud
  • Hired tow truck to get Home Depot truck out of mud
So, now I am drinking strongly spiked hot chocolate. Somewhere in the above process I lost my car keys again. Boo.
I'd like to build a radio. Perhaps several. AM then FM. Receiver. It's just something I'd like to learn more about, but also I feel ridiculous with a degree putatively in Electrical Engineering but without having ever constructed any radio device more complicated than a crystal set. It seems like there ought to be a nice book out there somewhere with progressively more sophisticated designs to play with?

One little idea I had was to try to build a clock would listen to and decode the time signals broadcast by NIST; those are on nice integer fixed frequencies, so maybe something simple (with a crystal oscillator) would work? [livejournal.com profile] ioerror, Google came up with a link to some neat pictures on your flickr account in response to the query, "homemade fm radio receiver". What's the story behind this nifty little contraption?

March 2020

S M T W T F S
1234567
891011121314
15 161718192021
22232425262728
293031    

Style Credit

Syndicate

RSS Atom
Page generated Aug. 28th, 2025 07:15 pm
Powered by Dreamwidth Studios

Expand Cut Tags

No cut tags

Most Popular Tags