2018-10-05

2018-10-05 本文已影响0人 carpediemmlf

Physics: take measurements and capture behaviour in mathematical form
Deviations from mathematical form implies the need to revise something
ideas of planning, executing and assessing measurements
Progress in physics
$\xrightarrow[]{Change\ quantity} System \\ \xrightarrow[]{Effect} Device/transducer \\ \xrightarrow[]{} Signal\ handling \\ \xrightarrow[]{} Data\ Recording$
Insights

Transducer should not affect system/device seen
Track changes easily
Deal with noise

Equivalent output circuit: ideal voltage source + output impedance
Equivalent input circuit: ideal voltage metre | input impedance

$V_{in} = V_{1} \frac{Z_{in}}{Z_{in} +Z_{out}}$

Desirable:

Transducer with low output impedance
'scope with high input impedance

Compensate the transducer with a complex impedance probe to avoid degrading of $V_{in}$ at high f
For other measurements:

Current: small $Z_{in}$ for 'scope
Power: $Z_{in} = Z_{out}$ to get the same maximum power measurable

Operational amplifier: high gain, high input impedance
For an ideal OP:

$A = \infty$
$Z_{in} = \frac{\partial V_{in}}{\partial i_{in}} = \infty$
$Z_{out} = \frac{\partial V_{out}}{\partial i_{out}} = 0$

Golden rules for an ideal OP:

GR1: $i_{+} = 0 \mathrm{A} = i_{-}$
GR2: $V_{+}=V_{-}$ if not saturated (proof by considering the non-inverting amplifier to calculate using current conservation and Ohm's Law)

Inverting voltage amplifier

Closed loop gain: $-\frac{R_{2}}{R_{1}}$

Non-inverting voltage amplifier

Closed lop gain: $1 + \frac{R_2}{R_1}$
Other persepctive: potential divider

NB:

Needs negative feedback
$V_{out} \leq +15 \mathrm{V}$ e.g. not saturated. After saturation

Make $Z_{2}$ frequency-dependent to perform filtering and selected amplification

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