Download 7B: Microphones, Filters, Oscilloscopes, and Amplifiers

7B: Microphones, Filters,
Oscilloscopes, and
Amplifiers
http://egweb.mines.edu/eggn250/exp7b.htm
Amplifier → makes a small
signal bigger
Filters
Op – amp
There are 2 amps in 1
side A is one amp, side
B is another
Orient with the notch up
Make sure +12 and –12 volts are in the
right place or you will ruin your
circuit!!!
Hints:
Wire your oscilloscope like last week
CH1 = Vin = V from function generator = the signal
you are trying to amplify
CH2 = Vout = V out of your circuit = the amplified
signal
Say… Vout/Vin = 10V
If Vin = 3V
Vout = 3V *10V = 30V
Do you really think you can get 30V out of your
system?
If you are supplying +12V the max you can get out
is 24V….
Use small input voltages !!! (but not so small that
you get allot of noise…)
For some more Op-Amp info, check out Phill’s page!
http://www.mines.edu/~pbradfor/melnotes.html
Bode Plot Examples:
http://www.mines.edu/fs_home/jmoss/7A.html
http://ee.utah.edu/~ee2100/Lect_1_16_02m.pdf
Y axis = 20Log(Vout/Vin)
X axis = f
To get log scale on the x axis
Plot just f
on x axis
Right click
on axis – format axis
Under
“scale” – check log scale
Janine’s helpful hints:
Experiment 7B -- Clarification & Guidance
Big Picture: This Part 2 of the 3-part lab.
7A – Part 1: We built & looked at the behavior of Lowpass, Highpass & Bandpass
filters.
7B – Part 2: We add amplification to the Lowpass & Highpass signals, via an Op-Amp.
7C – Part 3: We FINALLY add our Electro-Mechanical device, a Microphone. And
now you are prepared to both filter & amplify the output signal of the function generator
and microphone.
Sound exciting???!!! It will definitely be noisy that last week so bring your earplugs!
Experiment 7B – Corrections or Clarifications to Lab Manual
1) We don’t have sweep capability with the current Function Generator, so ignore
that part of the “Objectives”.
2) Add a 12 V Power Brick to the list of “Equipment & Supplies”
3) Hambley has moved Chapter 8 on Amplifiers to Chapter 11 and covers OpAmps
in Chapter 14. Chap 11 has more general info on amplifiers, while Chap 14 is
dedicated to the electronic opamp, which we will be using.
4) In the experiment write-up it suggests that you have used amplifiers in previous
experiments, particularly for the SG bridge output voltage. Sanity Check -Don’t be confused here. So far, you have not yet used an external hardware
amplifier in ANY of your circuits, i.e. on the protoboard. However, you have
been using the internal electronic amplifiers that are part of the DAQ board,
located in the computer (part of DAS: DAQ + Terminal Block + LabVIEW 6.1).
I will discuss these on Thursday – a good opportunity to point these out to you.
5) The connector color code they refer to in the experiment write-up is for the 5 pin
connectors that go to the power bricks. We’ll go over this in class Thursday.
6) In Figure 7B-3 the 0.01 F capacitor and 10 k resistor are in parallel and make
up the feedback loop. See Figure 12.21 in the Rizonni material we gave you
before Spring Break.
Experiment 7B – Some Info you’ll Find Useful
If you’re currently taking DCGN 381, this lab should work in fairly nicely with your
lectures this week on OpAmps & HW in Chapter 14. If not, review Chapter 14 in
Hambley or another EE text on the subject of OpAmps. The material we passed out to
you before Spring Break is from the Rizzoni text & is intended to supplement Hambley
as his Chapter 14 is weak on this subject.
I recommend visiting Jamie Turner & Phil Bradford’s homepages for some info on
OpAmps, Active Filters, Decibels & Bode Plots.
http://www.mines.edu/fs_home/jmoss/7B.html
http://www.mines.edu/~pbradfor/melnotes.html
Key Points about Amps & Op Amps:
I have gone through Chapters 11 & 14 in Hambley and tried to highlight the more
important points because I feel like it’s too much material to read, some may not cover
this in DCGN381 and you probably wouldn’t know what to study.
Amplifier (Amp) – Ideal amp produces output signal w/ identical waveshape as input,
but with larger amplitude.
vo (t )  Av vi (t )
where Av is the “voltage gain”
Inverting Amp – An inverting amp simply yields a negative value of Av, so the output
signal is inverted, in addition to being amplified. With an op-amp, the inverting input
terminals are marked with a negative (-) sign. See Figure 14.4, Hambley 2002.
Phase Shift – In reality the gain of an amplifier is not constant, but varies with frequency
and affects the phase of the input signal, as well as the amplitude.
Roll Off – The gain of an amplifier always drops off, roll off, at higher frequencies. This
is caused by small amounts of capacitance (C) in parallel with the signal path, or small
inductances (L) in series with the signal path.
Midband region – The wide range of frequencies where gain magnitude of an amplifier
remains constant. See Figure 11.21, pg. 488, Hambley 2002. We are using a DC coupled
op-amp, so the amp has constant gain all the way down to dc (freq=0).
Half-Power Freq (Corner Freq, -3dB Point) – This point of roll off is specified by the
half-power frequency, corner frequency, or –3 dB frequency.
V
Where output power ( V2) = 1/2 Amidband
Therefore, out  1  0.707
2
Vin
1  3.01dB
2
(Analogous to the filter circuit from last week). For this lab, this value can be estimated
based on the bandwidth and gain, as shown below.
In dB this becomes: 20 log
Differential Amps – Have 2 input sources. An ideal version of this amp would take the
difference of the 2 input voltages for Vin. See pg. 502, Hambley 2002. There is also a
common mode version, which averages the 2 input voltages for Vin.
Operational Amps (Op-Amps) – Today, it refers to integrated circuits that are
employed in a wide variety of general-purpose applications. They originated in analog
computer circuits to perform addition or integration of signals, therefore the name
operational. They are composed of field effect and bipolar junction transistors (FETs &
BJTs), Chapters 12 and 13 of Hambley 2002.
Open Loop Gain – The gain of the op-amp without a feedback network. Typically, opamps are used with feedback networks or a loop, as our circuits in 7B.
Closed Loop Gain – The gain of the circuit with the feedback loop (or network) in place.
Active Filters – the class of filters where an op-amp is included in the circuit design.
The op-amp can provide amplification, and options for inversion and averaging of the
signal, to name a few.
Experiment 7B – Help on PreLab Questions
I think you should have everything you need to work through the PreLab. You might get
stuck on the wiring diagrams.
Q3)
I believe if you use your Hambley text & the Rizzoni matl. you should be fine.
If you have questions email me, but not at the last minute.