AM Radio Receiver with Automatic Gain Control Unit Project Report (19.01.2015)

1. AM Radio Receiver with Automatic Gain Control Unit
Efe Arın#1
, Cem Recai Çırak#2
, Anıl Sönmez#3
, Cemalettin Taş#4
#
Electrical and Electronics Engineering, Middle East Technical University
Ankara, Turkey
1 e174048@metu.edu.tr
2 cem.cirak@metu.edu.tr
3 e174154@metu.edu.tr
4 e174157@metu.edu.tr
Abstract – This paper gives information about the procedure of
an AM radio receiver circuit implementation as EE313
(Analog Electronics Laboratory) course term project. First, a
wide range of simulations are run on SPICE Software.
Considering these simulation results, the prototype is set and
experimental results are attached. Comments on these results
and conclusion about whole implementation procedure is also
covered.
Keywords – AM Radio Receiver, Amplitude Modulation,
Envelope Detector, Automatic Gain Control, Audio Power
Amplifier
I. INTRODUCTION
Although there are few AM radio stations left around the
world, the broadcast over AM radio frequency is still in use.
Thanks to AM radio receiver circuit structure, these
frequencies turn into meaningful sounds. And AGC
(Automatic Gain Control) unit part provides stable output
sound level by adjusting the gain according to signal that has
been received. This paper follows the strategy as first
mentioning about full circuit block diagram of AM radio
receiver with AGC Unit, then analysing the individual parts
of this circuit. Both simulation results and experimental
results are also included in this paper.
II. AM RADIO RECEIVER
Since ease of use, the envelope detector receiver, which
consists of tuned filter, peak detector, RF amplifier and
audio amplifier, with integrated AGC unit are used. The
Block diagram of whole circuit is shown in Fig. 1.
Tuned
Filter
RF
Amplifier
Peak
Detector
Audio
Amplifier
Loud
Speaker
Antenna AGC Unit
Fig. 1: AM radio receiver with AGC unit block diagram
A. Antenna
AM broadcast frequency band lies between 530-1600
kHz as MW (medium wave) and 150-270 kHz as LW (long
wave) LW is taken to consideration in this project. Although
at least λ/4 antenna length should be used, the length
calculation (c/f) for LW band gives 1.5 km which is
impossible to implement. Therefore antenna length is
chosen about 1 meter.
B. Tuned Filter
This circuit decides which frequency will be used to get
audio from loud speaker by using second order passive
band-pass filters.
To get clear filtered signals, it does not needed to use
active filters as the frequency difference in those signals are
50 kHz (large enough). So passive filters are used in this
filter design and the gain above and under 15 kHz is
-20 db. In addition to this the quality factor did not taken so
high in order to not to miss the signal itself if the centre
frequency of the broadcast signal is not the exact desired
value. Same circuit diagram with the different values of
capacitors, inductors and resistors are used in this part. All
these there circuits can be seen in the Fig. 2 below.
Fig. 2: Tuned filters

2. The central frequencies of these filters are about 150 kHz,
200 kHz, 250 kHz. Values of the capacitors, inductors and
resistors are determined by using the central frequency,
bandwidth and quality factor formulas below seen in Eq. 1
and Eq. 2 and Eq. 3 respectively.
𝑓𝑜 =
1
2𝜋√𝐿𝐶
(Eq. 1)
𝐵 =
1
𝑅𝐶
(Eq. 2)
𝑄 =
1
𝑅
√
𝐿
𝐶
(Eq. 3)
Frequency response simulation results for each filter are
seen below in Fig. 3, Fig. 4, and Fig. 5.
Fig. 3: Frequency response simulation result for 150 kHz filter
Fig. 4: Frequency response simulation result for 200 kHz filter
Fig. 5: Frequency response simulation result for 250 kHz filter
C. Peak Detector
This part effectively detects the envelope. It is composed
of diode, capacitor and resistor. Diode will only allow
positive voltage and capacitor will hold the peak values of
this positive voltage. When peak value drops capacitor will
be discharging via resistor to lower value and when peak
value gets higher it will be charging.
The main issue with this circuit is the values of capacitor
and resistor. If we are to take time constant RC value very
high then time constant will be very high. Therefore,
capacitor will be discharging very slowly. This makes
capturing all the peak values impossible. Moreover, if we
take RC value very low then time constant will be very low.
This makes again capturing the peak values nearly
impossible. Because capacitor will be discharging very fast
even in between peaks which is what we do not want.
Values of resistor and capacitor are determined by using
the Eq. 4 and Eq. 5.
𝜏 = 𝑅𝐶 (Eq. 4)
1
𝑓 𝑎𝑢𝑑𝑖𝑜
≫ 𝜏 ≫
1
𝑓 𝑐𝑎𝑟𝑟𝑖𝑒𝑟
(Eq. 5)
The main idea is in the Fig. 6 below. A is the AM signal
and B is the detected envelope.
Fig. 6: Peak detector working principle
The peak detector part circuit diagram is shown below in
Fig. 7.
Fig. 7: Peak detector
D. AGC Unit
This circuit stabilizes the sound wave as an input which
is demodulated at the peak detector part, at its output. The
output voltage level is independent of its input and this is
done by the circuit by changing the gain of itself using
feedback mechanisms. Which means when the sound wave
has low peak voltage level the AGC unit increase its gain
and vice versa.
The circuitry implemented at this project is given in Fig.
8. At this circuit when output is stay at desired level the
lower part is inactive. At this situation AGC acts as a RF
amplifier and the upper part (R3 and R4) decides what the
gain is. When there is a change in output level basically the
lower part works as feedback mechanism. C2 is used as
coupling capacitor. PNP, R2 and C1 works as peak detector.
PNP rectifies the wave and C1, R2 create ripple. By this
circuitry part the wave is turned into DC voltage with the
voltage of wave peak value. This DC voltage applied to the
gate of NJF and NJF is used to adjust input voltage by using
its gate voltage dependent resistor characteristic. So the
output fed back by this lower part path.

3. Fig. 8: AGC unit
For example when output peak level decrease the DC
voltage at the gate of NJF decrease. So the resistance of NJF
increase which results in increase at the input voltage level.
Thanks to this feedback output voltage increased back to its
desired value.
AGC unit should be implemented before RF amplifier
since RF amplifier increase the difference between low and
high voltage levels and the signal becomes hard to be held
at the stable voltage level.
E. RF Amplifier
This part is basic voltage amplifier. Since the voltage
level of the AGC output is very low to directly send wave to
the speaker, there is a need for voltage amplifier. The circuit
used in this part is shown in Fig. 9.
Fig. 9: RF amplifier
Theoretical value of the gain is calculated as 11 by the
formula in Eq. 6.
𝐴 𝑣 =
𝑣 𝑜𝑢𝑡
𝑣 𝑖𝑛
=
𝑅1+𝑅2
𝑅1
(Eq. 6)
The transient analysis simulation result of RF amplifier
circuit is seen in Fig. 10.
Fig. 10: RF amplifier simulation result
F. Audio Amplifier
This part is used to increase power of the signal to get a
sound from the loud speaker. Although RF amplifier
increases the voltage sufficiently, it cannot provide enough
current. So there is a need for audio amplifier.
A class power amplifier has no distortion but its
efficiency is less than 30% in practical. And B class has
distortion at its dead zones due to high biasing voltages of
BJTs. AB class on the other hand use B class with diodes to
lower distortion by decreasing the bias voltages. Therefore,
AB class is used in this projects with lower distortion in
compared to B class and higher efficiency in compared to A
class.
The circuit diagram of the AB class power amplifier
used is shown in Fig. 11.
Fig. 11: AB class audio power amplifier
The simulation results according to input and output
(speaker) currents is given in Fig. 12.
Fig. 12: AB class audio power amplifier simulation results
G. Loud Speaker
Low resistant (8Ω) loud speaker is used. Although the
power of its output sound is not so high, the remaining part
of AM radio receiver will supply powerful enough signal to
listen. Additionally the low power loud speakers are cheaper.

4. III. RESULTS
Implementation of the AM radio receiver circuit setup is
seen in Fig. 13. The numbers given in the figure are
indicated as follows:
1. Antenna
2. Tuned Filter
3. Peak Detector
4. AGC Unit
5. RF Amplifier
6. Audio Amplifier
7. Loud Speaker
Fig. 13. AM radio receiver setup
In this experimental setup we faced with some problems
about peak detector, AGC unit and audio amplifier parts.
In peak detector part, we noticed that our calculated
theoretical time constant value was not valid in practice. To
overcome this problem, we tried to use a potentiometer
instead of the resistor. However we could not find the actual
time constant value. Therefore we obtained a triangular
waveform instead of a perfect sinusoidal wave at the output
of peak detector.
Since our AGC unit simulation results were not
meaningful, we did not use this part in our experimental
circuit. So, we bypassed the AGC unit part and used directly
the output value of the peak detector part as the input of RF
amplifier.
In audio amplifier part, we faced with some serious
heating problems due to high current values passing through
our BJTs when high peak voltage values which is larger than
1 volts were applied to the input. Therefore our input range
was limited by this heating problem of the BJTs.
IV.CONCLUSION
The final works about AM radio receiver with AGC unit
project are mentioned in this paper. Full circuit models are
given above. A lot of important concepts are learnt with this
project such as amplifiers, feedback systems, filters.
Theoretical and experimental results were not exactly same
as expected. But there were not big differences which can
be negligible.