Analog Electronics AGH Exercices

5 Power amplifiers

Exercise 1

The goal of our work is to develop a power amplifier which should display following parameters:

1. Input resistance Re = 100kΩ.

2. The load of this amplifier constitutes an activator. Its input resistance R_L=4Ω.

3. Output power should reach Ps= 20 W for a sinusoidal input signal of the rms value= 100 mV

4. Supply voltage for this amplifier is symmetric: E =15V et -E = -15V

5. The bandwidth of this amplifier is fixed as to be : 0-100 Hz

1.1. Solution I: use of operational amplifier (OP)

The examined setup is show an in Fig.1 The OP displays the typical parameters .The maximal output current i_amax=20 mA

Figure 1

The capacity C is used in order to limit the frequency bandwidth. Circuit C4, R3, acts as a filter:

1. Find the transfer function H(jω) =V_a/V_e; What is the type of this filter

2. Find the gain G_v1 of this filter , within its frequency band

3. What is the cut off frequency f_c1 of this filter? Propose le C and R3 in order to fulfill the requirements

4. What is the input resistance of amplifier in its frequency band . Propose R4 in order to fulfill the requirements

1.2.

For the following the following is to be considered.

• Input signal s_m(t)=V_e is mono frequency signal of frequency f=ω./2π::

• for f <fc the output signal becomes :

The resistivity R. is connected to the output.

Compute:

1. Instantaneous power P_ia s function of V_a and R

2. Mean output power P_a as function of V_am and R and as a function V_{a rms} and R

3. Taking into account the OP parameters maximal value of P_a and corresponding minimal value of R

4. Is it possible to fulfill the requirements using this circuit if R=R_L ?

2.1. Solution II. OP with a power follower (push pull class B)

Analyze the circuit from Figure 3.. The transistors T1, T2 s are the power transistors: i_c max = 10A. As well assume that threshold of BE junction is: V_BE = 0.6V. Illustrate the temporal forms of V_e,V_a, i_T1,i_T2,i_s,V_s . What is a main drawback of this solution

Figure 3

2.2. Solution III. OP in cascade with a power follower in AB class

Analyze the circuit from Figure 4 ..

1. Illustrate the temporal forms o V_e,V_a, i_T1,i_T2,i_s,V_s .

2. Find V_{sm max} and corresponding output power Ps.Is it possible to fulfill the requirements now  ?.

3. Find minimum value of RL and corresponding maximal value of Ps

4. Find the gain of the follower G_v2 =V_s /V_a

Figure 4

2.3. Final solution: class AB with a feedback

The circuit is modified as follows. (Figure 5)

Figure 5

1. Find expression for total tension gain G_{v tot}. =V_s /V_e

2. Find G_{V tot} that fulfills the requirements. Propose the values of R1 et R2

For the amplifier as above find :

1. Its current ,tension and power gains

2. Power supplied by +E and –E

3. Power efficiency

4. Power dissipated on each transistor T1 and T2

3. Propose the thermal resistance for radiators R_tR if:

Thermal resistance of T1 and T2 without radiators is R_tt=15 deg/W

maximal temperature of junction T_j=50°C and ambient temperature for this amplifier is T_a=20°C

4 Propose the circuit which will display the mean power P_s dissipated in the load. Draw its scheme; use any devices you need, give their characteristic values if they are significant.

5 Two amplifiers same as considered in (1, 2,3,) are connected as shown in fig.6

G=-1. What is the output power now

G=1. What is the output power now.

1. Figure 6

6. Does the amplifier from Fig.5 is a DC amplifier. ?

For the next questions we consider that the input signal V_e is taken as a constant DC signal :

and that G_vtot =10

6. Let us assume that maximum output tension │V_smax │=│0.9E. Illustrate the curve Vs=f(V_eo) for V_eo varying from -2 to 2 V. What happens when │Vs│> │V_smax│.

7. The input of the amplifier was modified, as illustrated in fig 7. The input signal is referred now as V_e1. The threshold tension of both diodes are V_d=0.6 V. (Assume that dynamic resistance R_d=0)

Figure 7

1. The input signal is set as

Illustrate, the temporal forms of V_e(t),V_s (t), i_s(t),

1. What is the function of above circuit?.

6. The circuit from fig 7 is removed . V_e(t) assumes now the recnagular waveform type periodic signa as shown in Fig.8; V_em=1V, period T=2s. Illustrate V_e(t),V_s (t), i_s,(t)

Figure.8

Find expression and give a value of:

• Mean output power P_s as function of V_e1m , G_vo and R_L

• Power P_a supplied by +E and –E

• Power amplification efficiency η

• How should be modified the amplifier in order to maximize η

• What is the class of such operating amplifier ?

10 Two transistors T1 and T2 are protected by fuses. Propose the circuit that lights a LED diode when a fuse is blown. Draw its scheme; use any devices you need, give their characteristic values if they are significant. Assume here that lighted LED diode needs:

-be forward polarized,

-current I_d≈10 mA ; if I_d>20 ma diode is damaged,

-tension U_df=3V.