Lab2

Project - EE 420L

Authored by Allan Pineda
pineda3@unlv.nevada.edu
May 03, 2017

Project Description: Design a transimpedance amplifier (TIA) usng either the ZVN3306A or ZVP3306A (or both) MOSFETs and as many resistors and capacitors as needed with a gain of 30lk. The design must be as high as possible driving a 10k load with as large of output swing as possible. AC coupling input and output is okay as long as the design can pass a 100Hz inout current. The report, in html, should detail the design considerations, and measured results showing the TIA's performance.

Design Considerations:

A transimpedance amplifier convert an input current to a voltage output. To obtain high gain of 30k, a push-pull amplifier will be use in this project. However, since this topology have a draw back of consuming too much current, additional resistor will be place to the source of both PMOS and NMOS. Figure below shows the schematic of the push-pull amplifier with and without resistor to limit the current.

Figure_1

Push-Pull Amplifier with No Resistor Schematic Push-Pull Amplifier with Resistor Schematic

One of the most important aspect in designing an op-amp is the gain. There are several choices of design can be implemented to get a good gain. However, the most suitable design to use in this project is the push-pull amplifier because of its simplicity, efficiency, low distortion, high output power and its large gain. In contrast, the push pull amplifier can draw alot of current from the source that cause the transistor to get really hot during operations. One way to solve this problem is to put a resistor in the source of both mosfet to limit the current consumed by the transistors.

Theoretical Gain Calculation:

The transistors use in this experiment is ZVP3306A and ZVN3306A. This transistor has transconductance of 7.2 mA/V for NMOS and 7.79 mA/V for PMOS with a set of current around ID = 209 microAmp. Current requirement is no more than 0.3mA; so to have room for little variation a approximately 209 micorAmp current was chose to avoid exceeding the current limit. The resistor value I chose was R8 = 10k ohms for PMOS and R2 = 10k ohms for NMOS. Resistor value of 33k ohms was chosen to have gain of more than 30k ohms. Hand Calculation below show the gm's values as well as the gain when a 9V DC power supply and AC of 100mV is use.

Simulated Gain:

Figure_2

Schematic Simulating the Gain Resulting Gain

Notice from hand calculation above, the gain is approximately 32k ohm using a 33k value for Rf resistor. It clearly shows Rf dictate the gain in the design push-pull amplifier above. Looking at the phase angle of simulated results, the phase is around 180 degrees indicating the negative sign in hand calculation. As mentioned above, R8 and R2 are added in the circuit to control the current consumed by op-amp.

Experimental Gain

Figure_3

Gain at 100Hz Gain at 1kHz Gain at 10kHz

Gain at 100kHz Experimental Graph
Table_1

		Frequency	Gain (Vout / IR(4))
Yellow	Output Voltage	100Hz	30k ohms
Blue	Input Voltage	1kHz	31.3k ohms
Red	Math (Blue-purple)	10kHz	31.3k ohms
Purple	Voltage After R4(10k ohms)	100kHz	20k ohms

Table_1 above shows the gain in varying the frequency from 100Hz to 100kHz. At 100Hz the gain in my design is still relatively high but when the frequency is in below 100Hz and above 100kHz frequency the gain will start to decrease. Comparing the experimental values to the simulation values, the resulting gains are similar but the bandwidth where the gain start to decrease are not the same this is due to other parameter included in model.txt files use in Ltspice simulation.

Power Dissipation:

The power dissipation for this design is around 1.97m watts. This is because of the resistor place in the drain of the PMOS and NMOS to control the current flow in the circuit.

Hand Calculation

Experimental Result for Power Dissipation:

Voltage Source of PMOS	Voltage Source of NMOS	Voltage Gate of both Transistor

Simulated Output Swing:

Using a 9V power supply and 1V amplitude sinusoidal input the output swing simulation results is around 1.03V for this design. See Figure_4 below.

Figure 4

Output-Swing Schematic Output-Swing Results

Output-Swing = 2.91V-1.88V = 1.03V

Experimental Output Swing

Output-Swing

The image above shows a 1V per division. Estimating the output swing, one can telll that the output swing is approximately 1.7V.

Conclusion:

The transimpedance amplifier is easy to design using push-pull amplifier. In push-pull amplifier a high gain can easily obtain by varying the feedback resistor. The higher value of the feedback resistor the larger the gain. However, the current consumed by the push-pull must take into consideration to avoid over heating transistor. The speed of the design can be dictated depends on the capcitor use. The bigger the capacitor the longer it takes to charge up hence the slower the speed of the design. Use small capacitor to get high speed design if needed.