Download Welcome to Bioinstrumentation Design

Amplifiers: From Ideal to Real
• Why amplify?
• Gain = Vout / Vin = sensitivity
• Ideal Op Amp
• Negative gain summation amplifier
• Positive gain for increased Rin
• Differential amplifier circuit
• Gain-bandwidth product; CMRR
• Nonlinear gain: The ideal diode
• Instrumentation Amp
• Isolation Amplifier
References:
•Horowitz & Hill, The Art of Electronics, Cambridge Univ Press
• James K. Roberge, Operational Amplifiers: Theory & Practice
can download at:
http://web.mit.edu/klund/www/books/opamps181.pdf
• Burr-Brown HANDBOOK OF OPERATIONAL AMPLIFIER APPLICATIONS
can download at:
http://focus.ti.com/lit/an/sboa092a/sboa092a.pdf
If Rf = 1000*Rs and G is 106,
then Vin is in the range of 1mV,
a so-called virtual ground.
Summation
Input impedance of NGSA
• Input resistance = Vin/Iin = R1= Rs ≈ 1KΩ
• Way less than the 1012Ω of the op amp itself
• NG: R1 may become voltage divider with
sensor output resistance
• Impedance includes frequency-dependent
current-voltage relationship e.g. V(jω)=jωC*I(jω);
impedance is a complex number
mechatronics.mech.northwestern.edu/design_ref/electrical_design
Unity Gain Voltage Follower
A good circuit for isolation
http://ytdp.ee.wits.ac.za/cctpng/Follower.png
Positive gain single input op amp ckt
If Rs = 0 then Vout = Vin, UGVF
The positive gain amplifier cannot
have a gain less than 1.
It can have only one input.
Differential amplifier with external resistors
Current-voltage relationship of a diode
• Where kT/q is about 25mV at room temp
http://keywon.com/wiki/images/
http://electricalandelectronics.org
Ideal diode / half wave rectifer
• When VIN > 0 volts output current can flow through the
diode and does so until Vin = Vout. (low Rf, like UGVF)
• When Vin < 0 the the diode looks like a high resistance, the
op amp output saturates at the negative supply voltage, and
the left resistor "pulls down" VOUT until it's zero.
Log amplifier
http://www.regentsprep.org/Regents/math/algtrig/
ATP8b/exponentialFunction.htm
Absolute value / Full wave rectifier
www.eleinmec.com/
www.physics.udel.edu/~nowak/phys645/More_opamp_circuits.htm
A compression function of input: suppresses large spikes on input
Exponential (anti-log) amplifier
www.corollarytheorems.com/Design/opamp.htm
A non-linear amplifier used to emphasis positive spikes in waveform; makes sense only when Vs > 0.
… getting the signs of Vin and Vout figured out…
Op Amp as integrator
Where we know a capacitor’s current-voltage relationship is (from Laplace transforms)
GBWP and CMRR (LF353)
dB = 20*log10(out/in)
(Implies first order LP filter in op amp)
Instrumentation Amplifier AD524
• No external components needed
• pins to select gains of 10, 100, 1000.
• GBWP 25MHz
• Common mode rejection ratio of 120 dB
• Data sheet:
www.analog.com/static/imported-files/data_sheets/AD524.pdf
• p. 4: differential resistance 109Ω
• p. 8: Absolute Maximum Ratings
dimensions in mm
• p. 10: CMRR (freq)
Gain (freq)
• Settling time: 10μsec
• p. 18: Ground, analog and digital; ground mecca
•www.analog.com/en/other-products/militaryaerospace/ad524/products/product.html#ppa_print_table
Isolation Amplifier AD203
• UGVF
• optical isolation; (wireless isolation)
• transformer isolation
• www.analog.com/en/amplifiers-and-comparators/isolationamplifiers/ad203/products/product.html
•Data sheet:
www.analog.com/static/imported-files/data_sheets/AD203SN.pdf
• Modulator is a Volt→Freq converter on input, and “tach” on output
• single +15v supply
• input impedance 1012Ω
• low GBWP about 1MHz
• CMRR about 120 dB
Iso Amp block diagram
Op amp as example of negative feedback
• When we come later to the topic of
automatic control by negative feedback,
we will see the op amp as a good example