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Old: Biophysics II - Labs > Amplifier > Flashcards

Flashcards in Amplifier Deck (19)
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1
Q

What is an amplifier?

A

electronic device used to increase the P of the signal (Pout > Pin)
→ additional Pout is provided by a P supply

2
Q

Explain the 2 types of transfer characteristics.

Give an example for each type.

Draw graphs.

A

the relationship btw input and output signal of the amplifier

Can be either:

  • linear: shape of the output signal is similar to the input, thus it is free of distortions (if Uin is increased Uout increases proportionally)
    e. g. in potentiometers
  • non-linear: distorted Uout
    e. g. in transistors
3
Q

What is a potential (voltage) divider AKA potentiometer?

Explain the voltage of each resistor element using a formula.

What are its transfer characteristics?

A

circuit of 2 resistors connected in series, divides the input voltage (U) in the ratio of the individual resistances (R1, R2), but still same current (Ohm’s law):

  • U1 = U*R1/(R1+R2)
  • U2 = U*R2/(R1+R2)

linear transfer characteristics

4
Q

What is gain?

Give the formula for power gain and voltage gain.

A

ratio of the output and input amplitudes

  • power gain: AP = Pout/Pin
  • voltage gain: AU = A = Uout/Uin

​always > 1, otherwise no gain, but loss

5
Q

Explain the power-voltage relationship in a formula.

A

P = U2/R

6
Q

What is the decibel scale?

A

logarithmic unit of the gain level is the bel, but the decibel (1/10 of a bel) is widely used

7
Q

Give the formula for the gain level (both power and voltage).

A

n = 10*lg AP = 20*lgAU
in [dB]

(because AP = U2out/Rin = Rout/U2in = A2U Rin/Rout <br></br>→ lg AP = lg A2U = 2 *lg AU)

8
Q

What is an amplifier with feedback?

Differentiate btw its 2 types.

A

fraction of the output of the amplifier is added back into the input

  • positive feedback = signal same phase as input
  • negative feedback = signal inverted
9
Q

How is an amplifier with negative feedback constructed?

Explain its function.

Draw it.

A

Parts:

  • summing circuit
  • basic amplifier
  • feedback loop (= either potentiometer/transistor)

Function:

  1. Uin feeds basic amplifier → gain level >> 1
  2. “gained” signal into feeback loop → feedback gain
  3. summing circuit inverts feedback signal back with opposite phase
10
Q

What is a feedback loop/circuit?

What is loop gain?

A

usually a voltage divider circuit is used as the feedback loop, which is built from resistors (cf. voltage divider):

Uout * β = Uout * R1/(R1+R2)

  • feedback “gain”: β = R1/(R1+R2) (always less than 1 obv)

loop gain: times of signal amplification * β

11
Q

The increased U input for the amplifier obtained by the feedback loop depends on.. ?

A

voltage divider(s) of feedback loop→ resistor elements

  • T independent
12
Q

What are the advantages and disadvantages of a feedback loop?

How can the disadvantage be compensated?

A

Advantage:

  • significantly decreased distortion of amplifier
    → increasing loop gain → increased dependence on linear transfer characteristics of potentiometer

Disadvantage:

  • gain with feedback is lower than initial gain (because β of feedback loop ≄ 1)
    can be compensated by adding several feedback amplifiers in series → equivalent gain = sum of gains
    BUT: transfer bandwith increased
13
Q

What are frequency response characteristics?

Draw a graph.

A

= frequency dependence of the gain level of the amplifier

14
Q

What are cut-off frequencies?

A

limits of the frequency interval of the amplifier, where the gain level is considered to be constant

lower flower/upper fupper cut-off frequency = nmax -3 dB (≙ Pout decreased by half, cf. decibel graph)

15
Q

What does it mean if fupper is increased?

A

enables distortion-free transfer of faster (= higher frequency) signals

16
Q

What is the transfer band in a frequency response characteristics graph?

What is its relevance?

A

frequency range btw the lower and upper cut-off frequency

⇒ frequency range with largest gain level
⇒ must be optimized to fit typical frequency region of signal to avoid distortion

17
Q

What are the properties of an ideal amplifier?

A
  • very high gain, high stability
  • broad transfer band (flower = 0 (DC), fupper = ∞)
  • linear transfer function (no distortion of the signal shape)
18
Q

What are the properties of an amplifier with negative feedback?

A
  • decreased gain (only disadvantage)
  • improved linearity of the transfer function, less distortion
  • improved stability
  • broadened transfer band
  • less disturbing effects if noise added to the output signal
19
Q
A

NOCH AUSSTEHEND