Lecture 11. MEASURING RECTIFIERS

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1 Technical University of arna Department of Electronics and Microelectronics ELECTONIC MEASUEMENTS Spring term 200 GIGO H.G., Assc. Prof. Dr. Еng. in Measurement Systems Lecture. MEASUG ECTIIES.. Superdiode. Precision half - wave ectifier The diode rectifier circuit and its associated voltage transfer characteristic curve are shown on igure (a) and (b). igure. Diode rectifier circuit (a) and voltage transfer curve (b) The offset voltage d is about 0.7 olts and this offset value is unacceptable in many practical applications. The operational amplifier and the diode in the circuit of igure 2 form an ideal diode, a superdiode, and thus they eliminate the offset voltage d from the voltage transfer curve forming an ideal half wave rectifier. Superdiode igure 2. Precision half wave rectifier circuit and its voltage transfer curve. Let's analyze the circuit by considering the two cases of interest: in >0 and in <0. or in <0 the current I 2 and I d will be less than zero (point in a opposite direction to the one indicated). However, negative current can not go through the diode and thus the diode is reverse biased and the feedback loop is broken. Therefore the current I 2 is zero and so the output voltage is also zero, out =0. Since the feedback loop is open the voltage I at the output of the op-amp will saturate at the negative supply voltage.

2 or in >0, out = in and the current I 2 =I d and the diode is forward biased. The feedback loop is closed through the diode. Note that there is still a voltage drop d across the diode and so the op-amp output voltage out is adjusted so that out = d + in. MEASUG ECTIIES (M) AEAGE ESPONDG MEASUG ECTIIES Т 0 =. dt Т Measuring rectifiers a) One wave noninverting configuration; b) One wave inverting configuration; c) Two wave rectifier Gain &Errors: the same as for Amplifiers or c) the both gains mast be equal:

3 2 / = (+ 5 / 4 ) AMPLITUDE ESPONDG MEASUG ECTIIES a) b) c)

4 d) Gain &Errors: the same as for Amplifiers.2. Practically circuit diagrams of Precision ectifiers = + 2, when 0, when < 0 > 0 igure 3. Precision half wave non-inverting rectifier circuit and its time-diagram and transfer function. = 22, when 2 0, when > 0 < 0 igure 4. Precision half wave inverting rectifier circuit and its time-diagram and transfer function = + = 22 2 igure 5. Precision full-wave inverting rectifier circuit and its time-diagram and transfer function.

5 + = ( ) max igure 6. Precision non-inverting amplitude rectifier circuit and its time-diagram and transfer function. + = ( ) max igure 7. Investigated precision non-inverting amplitude rectifier circuit and its transfer function. = ( ) pp igure 8. Precision pic-to-pic rectifier circuit and its transfer function..3. Active Low Pass ilters.3.. irst - Order Active Low Pass ilter. The Active Low Pass ilter using an electronic integrator is shown in ig.9, where the input voltage is: v = sinωt and the feedback impedance Z is: in A Z = + jω C.

6 ig.9 An Active Low-Pass ilter v in = sinωt Transfer function: out Z A ( ω) = = in Z and gain: A( ω) = out in = = + + jω C ( ω C) 2 As you can see, the diagram in ig.0 shows the logarithmic plot of gain A(ω) versus frequency. were ω H = C is called cut-off frequency ig.0. Bode plot of active low pass filter with gain of 5 At frequencies much less than ω H (ω<< ω H ) the voltage gain becomes equal to /, while at frequencies higher than ω H (ω>> ω H ) the voltage gain decreases at a rate of 20dB per decade Second - Order Active Low Pass ilter Circuits. The Second Order Active Low Pass ilter using an electronic integrator is shown in ig.(a) and (b): igure -a. auch s Inverting Low-Pass ilter Structure. igure -b. Sallen-Key s Non-inverting Low-Pass ilter Structure.

7 .3.3. Average responding full wave rectifier. + 2 = 5 4 = 5 8 out ina 4 7 igure 2. Average responding full wave rectifier arna, Autumn, 2009 Assoc. Prof. Dr Eng. Hristo Gigov