11. Amplifier

Question 1

What is an amplifier?

Answer 1

an electronic component that amplifies the signal connected to its input and provides an amplified signal on the output.

→ The electronic signal on the output has a greater power than that on the input

Question 2

Structures of an amplifier

Answer 2

Amplifiers can be built from vacuum tubes (valves), transistors or integrated circuits (operational amplifiers).

Question 3

The amplifying process is based on the input-controlled ___ of the electronic components of the amplifier.

Answer 3

change of the resistance

Question 4

What is a POTENTIAL (VOLTAGE) DIVIDER?

Answer 4

circuit of two resistors connected in series. It divides the input voltage (U ) according to the ratio of the individual resistances

Question 5

What is TRANSFER FUNCTION?

Answer 5

the relationship between the input and output signal of the amplifier

Question 6

What is the linear transfer function?

Answer 6

the shape of the output signal is similar to the input and free of distortions.

Question 7

the transfer characteristics of the transistor are ___

→ the signal shape of the output Uout = U1 is distorted compared to the input.

Answer 7

nonlinear

Question 8

the transfer characteristics of voltage divider is linear or nonlinear?

Answer 8

linear

Question 9

What is GAIN?

Answer 9

the ratio of the output and input amplitudes.

Question 10

What is power gain?

Answer 10

the ratio of the output and input power

Question 11

What is voltage gain?

Answer 11

he ratio of the output and input voltage

Question 12

What is DECIBEL?

Answer 12

logarithmic unit of the gain level is the bel, but the decibel (one tenth of a bel) is more widely used.

Question 13

What is Distortion?

Answer 13

The amplitudes and phases of the Fourier components of the input signal change when passing through a nonlinear amplifier.

→ the shape of the output signal differs from the shape of the input signal.

Question 14

What is gain level?

Answer 14

Power gain or voltage expressed in decibel unit

Question 15

PROPERTIES OF THE IDEAL AMPLIFIER:

Answer 15

• 􏲳 very high gain, high stability
• broad transfer band
• linear transfer function (no distortion of the signal shape)

Question 16

PROPERTIES OF THE AMPLIFIER WITH NEGATIVE FEEDBACK:

Answer 16

• decreased gain
• improved linearity of the transfer function, smaller distortion
• improved stability
• broadened transfer band
• smaller disturbance if noise is added to the output signal.

Question 17

Properties of real amplifier

Answer 17

Question 18

What is Negative feedback?

Answer 18

a fraction of the output voltage of the amplifier is inverted and added back to the input

Question 19

NEGATIVE FEEDBACK

The voltage gain of the basic amplifier is A_U, and it is usually much greater than ___ (number)

Answer 19

1

Question 20

Describe AMPLIFIER WITH FEEDBACK

Answer 20

fraction of the output of the amplifier is added to its input.

→ If this signal has the same phase as the input, then it is called positive feedback

→ if it is inverted, then it is called negative feedback.

Question 21

What is FEEDBACK LOOP, FEEDBACK CIRCUIT

Answer 21

usually a voltage divider circuit is used as the feedback loop, which is built from resistors

→ it is a linear circuit element: U_out ·􏰵 = U_out ·R₁/(R₁+R₂).

Question 22

What is loop gain?

Answer 22

The product of the basic amplifier gain and the feedback gain

Question 23

the gain of the amplifier with negative feedback is determined by ___

Answer 23

the elements (R1, R2) of the feedback loop only.

Question 24

The gain of the amplifier with negative feedback is determined by the elements (R1, R2) of the feedback loop only.

→ Characteristics of these resistances.

Answer 24

These resistances are very stable, temperature independent, and, as a voltage divider, they have linear transfer function

Question 25

Disadvantages of amplifier with negative feedback

Answer 25

Because part of the output signal is subtracted from the input, the **gain with feedback (*****A*****_U,FB ) is lower** than the initial gain (*A*_U). (→ This decrease can be compensated easily by connecting several feedback amplifiers in series, one after the other. The equivalent gain of amplifiers connected in series is the product of their gains (or their sum in dB unit).)

Question 26

What is FREQUENCY RESPONSE CHARACTERISTICS

Answer 26

frequency dependence of the gain level of the amplifier.

Question 27

FREQUENCY RESPONSE CHARACTERISTICS → Are the descending parts of the curve at low and high frequencies typical features of all amplifiers?

Answer 27

Yes

Question 28

What are LOWER AND UPPER CUT-OFF FREQUENCY (FREQUENCY LIMITS

Answer 28

limits of the frequency interval of the amplifier, between which the gain level is considered constant. → By convention, the lower and upper frequency boundaries of the frequency response at the *n*_max –3 dB gain level are defined as the lower ( *f*_lower ) and the upper ( *f*_upper ) cut-off frequencies respectively.

Question 29

What is transfer band?

Answer 29

frequency range between the lower ( *f*_lower) and upper ( *f*_upper) cut-off frequencies of the amplifier. Outside the transfer band the gain level is variable and smaller than within the band.

Question 30

What happen if we Increase of the upper cut-off frequency ( *f*_upper)?

Answer 30

The power of the output signal decreases by half

Question 31

Negative feedback **significantly decreases \_\_\_** which arise from changes in the gain *A*U of the amplifier.

Answer 31

**The effect of disturbing signals**

Question 32

What are the advantages of negative feedback in the amplifier?

Answer 32

These **resistances are very stable**, temperature independent, and, as a voltage divider, they have **linear transfer function**. → the **gain of the basic amplifier (and its nonlinearity) does not play a role** any more → the initial **distortion** of the basic transistor amplifier shown in Fig. 3. **is reduced significantly**. → The greater the loop gain (*A*U*·*􏰵 \>\>1), the greater is the effect.