AM 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 receiver circuit implementation as EE313 (Analog Electronics Laboratory) course term project. First wide range of simulations are run on LT Spice. Considering these simulation results the prototype is set and experimental results are attached. Comments on these result and conclusion about whole procedure is also covered.

Keywords — Radio, AM Receiver, Tuned Filter, 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 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 follow the strategy as first mentioning about full circuit block diagram of AM 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 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 Amplif ier

Peak Detector

Audio Amplif ier

Loud Speaker

Antenna AGC Unit

Fig. 1: AM 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

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.

√ (Eq. 1)

(Eq. 2)

(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.

(Eq. 4)

≫ ≫ (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.

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.

(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 receiver will supply powerful enough signal to listen. Additionally the low power loud speakers are cheaper.

III. RESULTS

Implementation of the AM 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 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 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.