Op Amp Circuits

Op-Amp CircuitsBE201 Lectures 4Lab 2 prelab you will need to download PSPICE See Moodle1New material: Biomedical Measurement SystemBiosignal SourceSensor /TransducerSignal ConditioningA/D conversion Digital Signal & signal processingData Storage Display

(sensor)An ECG is a biosignal where a voltage is generated on the surface of the body.A surface electrode is the transducerWe need a way to amplify the signal Op Amps are sometimes involved.

Analog Signal ConditioningMost biological signals are weak and contain a lot of noiseRecall: the magnitude of an ECG signal; ~ a few mV__________________: increasing the dynamic range (the ratio between the largest and smallest possible values of a changeable quantity)________reducing the noise

___________shifting signals

Analog Signal Conditioning Example: Original SignalAssume acquired raw signal is: Dynamic range of 0.05-0.15 V5 Hz signal + high frequency noise

(V)Amplification

Amplify signal with a Gain = 20 The dynamic range of : 0.05-0.15 V => becomes 1-3 V(V)

Op-ampA powered op-amp without a feedback circuit has output of +Vsat, -Vsat, or oscillating between the two.

To maintain a useful Vout between +Vsat and -Vsat we need a feedback circuit.

For standard (negative feedback) op-amp circuits,No current flows into either input terminal of the op-amp (i.e., they have an extremely high impedance)

I- = I+ = 0 or Ipin3 = Ipin2 = 0

Infinite gain: positive and negative input terminals are at the same voltageV- = V+ or Vpin3 = Vpin2

Op-amp Symbol 8-pin 741 op-amp

Negative feedback

Op-amp inside

741 uses only 20 transistors7Unity Gain Follower = buffer

Keys to analyzing op-amps(infinite gain)Voltage at 2 and 3 are the sameHigh impedance) No current goes into 2 or 38Inverting Amplifier

Closed loop gain = ACL 9Non-inverting Amplifier

10

Mystery Circuit. What does it do?

11Differential Amplifier (ideal)

Can Ri2 and Ri3 be exactly the same? (Are there any exact measurements?)

Can Rf3 and Rf2 be exactly the same?12

Common mode / differential modeVoltages:VDM = V1 V2VCM = (V1 + V2) / 2

V2V1CMRR: Common-mode rejection ratio

dB = 20(log (ratio)) 90 dB means 20* Log of the ratio; or log the ratio = 4.5; or about 30,000 times14Op-Amp CircuitsBE201 Lecture 415