Amplifier

Question 1

What is an amplifier?

Answer 1

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

Question 2

Explain the 2 types of transfer characteristics.

Give an example for each type.

Draw graphs.

Answer 2

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 U_in is increased U_out increases proportionally)
  e. g. in potentiometers
• non-linear: distorted U_out
  e. g. in transistors

Question 3

What is a potential (voltage) divider AKA potentiometer?

Explain the voltage of each resistor element using a formula.

What are its transfer characteristics?

Answer 3

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

Question 4

What is gain?

Give the formula for power gain and voltage gain.

Answer 4

ratio of the output and input amplitudes

• power gain: A_P = P_out/P_in
• voltage gain: A_U = A = U_out/U_in

​always > 1, otherwise no gain, but loss

Question 5

Explain the power-voltage relationship in a formula.

Answer 5

P = U²/R

Question 6

What is the decibel scale?

Answer 6

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

Question 7

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

Answer 7

n = 10*lg A_P = 20*lgA_U
in [dB]

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

Question 8

What is an amplifier with feedback?

Differentiate btw its 2 types.

Answer 8

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

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

Question 9

How is an amplifier with negative feedback constructed?

Explain its function.

Draw it.

Answer 9

Parts:

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

Function:

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

Question 10

What is a feedback loop/circuit?

What is loop gain?

Answer 10

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

U_out * β = U_out * R₁/(R₁+R₂)

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

loop gain: times of signal amplification * β

Question 11

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

Answer 11

voltage divider(s) of feedback loop→ resistor elements

• T independent

Question 12

What are the advantages and disadvantages of a feedback loop?

How can the disadvantage be compensated?

Answer 12

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

Question 13

What are frequency response characteristics?

Draw a graph.

Answer 13

= frequency dependence of the gain level of the amplifier

Question 14

What are cut-off frequencies?

Answer 14

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

lower f_lower/upper f_upper cut-off frequency = n_max -3 dB (≙ Pout decreased by half, cf. decibel graph)

Question 15

What does it mean if f_upper is increased?

Answer 15

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

Question 16

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

What is its relevance?

Answer 16

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

Question 17

What are the properties of an ideal amplifier?

Answer 17

• very high gain, high stability
• broad transfer band (f_lower = 0 (DC), f_upper = ∞)
• linear transfer function (no distortion of the signal shape)

Question 18

What are the properties of an amplifier with negative feedback?

Answer 18

• 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

Answer 19

NOCH AUSSTEHEND