INVESTIGATION of noise in amplifiers operating in gain compression Undergraduate Research Presentation (EPSCoR) Spring 2010 Department: Electrical and Computer Engineering Date: 24th April 2010 Venue: University of Wyoming Classroom Building Rm 115 Prepared by: Chepchumba Soti Limo Advisor: Dr. Eva Ferre-Pikal

HIGHLIGHTS Introduction Background Objectives Simulations Measurement Systems Results Conclusion

INTRODUCTION: Noise: According to Motchenbacker and Connelly, noise is “any unwanted disturbance that obscures a desired signal. All signal that are transmitted go through some processing after they have been received All signals become weaker as they are being transmitted therefore necessary to amplify signal after receiving it Unfortunately, amplifiers add noise to the signal therefore degrading the system performance.

Types of Noise Phase Noise: Fluctuations at the zero crossings Amplitude Noise: Fluctuations at the peaks

BACKGROUND Transmitters and receivers have oscillators A basic oscillator consists of an amplifier and a filter connected in a closed loop The amplifier in the oscillator is the main source of noise when the oscillator is operating in very high frequencies Gain Compression Region This is the region where the amplifier is no longer linear i.e. the output power is no longer proportional to the input power Why are we interested in studying amplifiers operating in gain compression? Most amplifiers in the oscillator operate in gain compression

OBJECTIVES For this project the common emitter (CE) amplifier shown below was built. The main aim of this project is to see how varying the value of the inductor L1 (20nH and 100nH) will affect the PM noise of the amplifier when operating in gain compression . All measurements were taken at a frequency of 500MHz.

SIMULATIONS Before proceeding to build the circuit, we had to make sure that the DC bias of the amplifier was just right i.e. that we were not passing to much current through the transistor to destroy it. Waveform of the magnitude of the gain (Vo/Vi) in the frequency range 10 KHz to 10 GHz

Waveform of the phase of the gain (Vo/Vi) in the frequency range 10 KHz to 10 GHz Apart from making sure our amplifier is biased just right, the simulations help us know how much gain we can expect when we take our measurements. That is what the graphs in this sections represent.

MEASUREMENT SYSTEM Dynamic Signal Analyzer SR785 Instruments used: Agilent E360A Tripple Output Supply Provided the DC supply needed to power the amplifier 6062A Synthesized RF Signal Generator Provided the input AC signal at 500MHz Dynamic Signal Analyzer SR785 Used for injecting the noise into the circuit, measuring the phase noise and measuring the low frequency noise at the collector and the base Giga-Tronics 8541C Universal Power Meter Used for measuring the input and output power of the system LeCroy 9310M Dual 300Mhz Oscilloscope Used for callibrating the amplifier which helped in calculating the actual phase noise of the circuit

Using the information gotten form the measurements taken from the instruments mentioned previously, we were able to measure the total noise in the amplifier, and since we injected a known low frequency noise signal into our amplifier, we were ale to calculate the actual phase noise that the amplifier injects into the overall system. The low frequency noise that was injected into the system was a sinusoidal signal with an amplitude of 50Vpk and a frequency of 50Hz.

RESULTS We started by characterizing the amplifier so that we can identify where exactly the 1dB, 2bB and 3dB compression points are. This is because this are the three points at which we are going to take readings at and compare. We started with the 100nH inductor and the characterization graph is shown below.

After characterizing the amplifier, we were able to establish the 1dB, 2dB and 3dB points for the 100nH inductor as follows: 1dB => Pin = -18.79dB 2dB=> Pin = -15.69dB 3dB=> Pin = -14.11dB Once we knew what the exact input power needed for the CE amplifier, the system for measuring the phase noise was set up and calculations done. A summary of the results are listed as follows: @ 1dB Phase Noise = -87.97dB @ 2dB Phase Noise = -87.44dB @ 3dB Phase Noise = -93.48dB

CONCLUSION Because of time and all the obstacles we have had to overcome in building the circuit and taking measurements, we have just been able to successfully complete the first phase of this project. However, since the circuit is already built and the measurement system is already set up, taking measurements for the second phase of this project should not take nearly as long as the first phase. The most time consuming part will be changing the inductor. With the results we have obtained so far, they seem to be consistent with the theory behind this project. Since the main objective is to compare the results we have obtained with the ones we are yet to get, I am unable to do any comparison. It is for this reason that I do not have a conclusion now.

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