Lab

Project Report - ECE 420L

Authored by Stephanie Silic

silics@unlv.nevada.edu

May 2nd, 2017

Lab Description

This project presents a Transimpedance Amplifier (TIA) design using a push-pull amplifier.

A transimpedance amplifier takes a low-current output, often the output of diodes or other sensors, and converts the small, hard-to-measure signal to a larger voltage which is easier to analyze.

Our specifications for this amplifier design are the following:

Design a TIA with

a gain of 30,000
As fast as possible driving a 10k load, with good output swing
Can pass a 100Hz input current
Can draw under quiescent conditions ( no input signal), no more than .3mA (300uA) from a 9V supply voltage. i.e. Quiescent power consumption is less than 2.7mW for any power supply

Report should include design considerations and measured res ults showing the TIA's performance.

Lab Report

A push-pull amplifier can be used as a transimpedance amplifier, since it has high gain, and can drive a load since it can also act as a buffer.

The first consideration taken into account in this design was that we must not draw more than .3mA through the circuit when the 9V power supply is on. In order to ensure that only this smal current flows in the circuit, we will add resistors to the sources of the PMOS and NMOS devices, (between the source and VDD of the PMOS, and between the source and ground of the NMOS) devices to limit the current.

In the push-pull circuit we are making use of (below), the resistor values can be selected as detailed below.

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Project/Push_Pull_ckt.PNG

The gain of a transimpedance amplifier is the voltage out divided by the current in (Vout/IIN). The capacitors are added to ensure we can treat the sources as AC ground, for the gain analysis.

We know the values of KP and threshold voltage from the Spice model (see Lab 6 ) for the ZVN3306A and ZVP3306A MOSFETs. From there, we can set the drain current to a value under .3mA, and solve for the VGS, VSG, and the resistor values, after fixing one resistor value (say, R3 in the schematic above).

Calculations:

KPn = 0.1233 Vthn = 1.824V
KPp = 0.145 Vthp = 2.875V

Using a value of 10k for the resistors connected to the sources of the MOSFETs will limit the current drawn by the circuit.

The final schematic is shown here:

The current through the circuit, with the input disconnected (the wire cut between C3 and Vg) is simulated to make sure it is near under .2mA as intended:

A gain of 30,000 is equal to 20log(30,000) = 89.5 dB. Simulating the frequency response of the circuit, a 30k resistor value for R4 was found to create an overall gain of 89 dB :

Transient Simulation:

(Note: V(v2) is multiplied by 5 so that it's value can be read more easily from the waveform.)

The gain can be estimated as160mV/ (60mV-6mV/ 10k ) = 160m/5.4u = 29.6 k.

Measured gain at 100 Hz:

http://cmosedu.com/jbaker/courses/ee420L/s17/students/silics/Project/Measured_gain_100Hz.PNG

The blue wave, Channel 2, is the input signal, amplitude 50mV. The purple wave (channel 3) is the signal across the 10k input resistor, with an amplitude of 11 to 13mV (11mV was used in the calculation seen below, but 13mV was also seen, as in the oscilloscope screenshot above).

The green wave (Channel 4) is the output voltage of the push-pull TIA.

The calculations for the gain of the circuit from the measured results is shown here:

Thus, an input current of 3.9 uA is converted to an output voltage of 125mV peak-to-peak at 100Hz. (The speed of the amplifier is determined below; the design operates with the same gain up to about 60kHz.)

The amplifier is drawing 235 uA from the power supply:

Speed of the TIA:

The gain of the amplifier was seen to begin dropping at about 60kHz:

Maximum Output Swing:

The output swing was found by increasing the amplitude of the input voltage (and consequently the input current) and looking at the waveforem when the output was clipped to its maximum and minumum values:

The output swing of this design is from about 4.5 V. This output swing may be able to increase with added PMOS and NMOS devices in parallel, with larger resistors to account for the greater current demand.

In conclusion, this lab has presented how the push-pull amplifier can be implemented as a transimpedance amplifier drawing very little current but working effectively at taking a small input current, and amplifying the signal to a readable output voltage.

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