Author: Doug Mercer, consultant researcher of ADI company, and Antonio miclaus, system application engineer of ADI company

target

The purpose of this experiment is to study the input stage configuration of simple transimpedance amplifier.

background information

The output voltage of the transimpedance amplifier is proportional to the input current. Transimpedance amplifiers are often called transimpedance amplifiers, especially as semiconductor manufacturers like to call them. In network analysis, the general description of transimpedance amplifier is current controlled voltage source (CCVs).

The inverted transimpedance amplifier can be composed of a conventional operational amplifier and a single resistor. A resistor is connected between the output of the operational amplifier and the inverting input, and the in-phase input is connected to ground. In this way, the output voltage is proportional to the input current at the inverting input node and decreases as the input current increases, and vice versa.

This experiment explores an alternative differential input structure, which can generate inherent low input impedance (current input). On the contrary, the voltage differential pair explored in ADI’s June student zone experiment and July student zone experiment (MOS) has a relatively high input impedance. A complete conversion amplifier may need to add more gain stages and an output driver stage.

Material Science

  • Adalm2000 active learning module
  • Solderless bread board
  • Jumper
  • Three 1 K Ω resistors
  • Two 2.2 K Ω resistors
  • A 47 K Ω resistor
  • Two 10 μ F capacitance
  • Two NPN transistors (2N3904 or ssm2212)
  • Two PNP transistors (2N3906 or ssm2220)

explain

The circuit and connection connected with adalm2000 (ADI company) are shown in Figure 1. NPN transistors Q1 and Q2 and PNP transistors Q3 and Q4 shall be selected from the available devices with the best VBE matching. Transistors manufactured in the same package, such as ssm2212, sm2220, or ca3046, tend to match better than individual devices. When exploring the working principle of this circuit, the oscilloscope input 1+ can be connected to the connection point of Q1 and Q3 emitters, or to the collector of Q1 or Q3. The current input node at the emitter connection points of Q1 and Q3 is nominally low impedance, so it can be driven from a current source. The AWG output of the adalm2000 is more like a voltage source. Therefore, the 1 K Ω resistor Rin is used to convert the voltage output of awg1 into current (iin = vin/1 K Ω).

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Figure 1 Current driven transimpedance amplifier input stage

poYBAGIcmo6AduBuAAIYzzR7EeM756. png

Figure 2 Current driven transimpedance amplifier input stage on bread board circuit

hardware setup

The first waveform generator W1 is configured as a 1 kHz sine wave with a peak to peak amplitude of 800 MV and an offset of 0. Channel 1 of the oscilloscope shall be connected to display the output of the first generator, and channel 2 shall be set to display the output signal (40 mV per grid).

Procedure steps

Configure the oscilloscope to capture multiple cycles of the two measured signals. Using LTSpice ® The waveform example of is shown in Figure 3.

pYYBAGIcmruAMZCJAANxSoEDv2M373. png

Figure 3 Waveform of input stage of current driven transimpedance amplifier

Observe the output of RL, which is the AC coupled sum of collector signals of Q1 and Q3. Measure the voltage gain from awg1 to RL and compare it with the calculated value. Observe the voltage amplitude of the signal at the current input node (where the emitters of 1+, Q1 and Q3 are connected). Based on this amplitude, the input current amplitude (voltage at both ends of RIN divided by RIN) and the effective input resistance of the amplifier are calculated. Compare these values with the calculated values.

Configure voltage drive

Additional materials

  • A 470 Ω resistor

explain

Now reconfigure the input to voltage drive. Replace Rin with 470 Ω resistor and ground the other end, as shown in Figure 4. Disconnect the emitters of Q2 and Q4 from ground and awg1 output.

poYBAGIcmsKAMPuTAAH9TJE71Jc903. png

Figure 4 Differential pair with tail current source

hardware setup

The first waveform generator W1 is configured as a 1 kHz sine wave with a peak to peak amplitude of 800 MV and an offset of 0. Channel 1 of the oscilloscope shall be connected to display the output of the first generator, and channel 2 shall be set to display the output signal (80 mV per grid).

poYBAGIcmsqAGvTfAAiHX1R3q6s002. png

Figure 5 Voltage driven transimpedance amplifier input stage on bread board circuit

Procedure steps

Configure the oscilloscope to capture multiple cycles of the two measured signals. An example waveform using LTSpice is shown in Figure 6.

pYYBAGIcmtyAd1DyAAR_ m9KQXDo994.png

Figure 6 Waveform of input stage of voltage driven transimpedance amplifier

Observe the output of RL, which is the AC coupled sum of collector signals of Q1 and Q3. Measure the voltage gain from awg1 to RL and compare it with the calculated value. Observe the voltage amplitude of the signal at the current input node (where the emitters of 1+, Q1 and Q3 are connected). Based on this amplitude, the input current amplitude (voltage at both ends of RIN divided by RIN) and the effective input resistance of the amplifier are calculated. Compare these values with the calculated values.

In this voltage driven configuration, in order to measure the current to be supplied by the input driver (W1), a 1 K Ω resistor should be inserted in series with awg1 (and the transmitters of Q2 and Q4). Connect differential channel 1 oscilloscope input 1+, 1-, across 1 K Ω resistance. When awg1 swings at ± 400 mV, observe the voltage and calculate the current.

Question:

  • State the main characteristics that define the transimpedance amplifier.
  • Can you point out some applications using such circuits? You can find the answer on the student zone forum.

About the author

Doug Mercer graduated from Rensselaer Polytechnic Institute (RPI) with a bachelor’s degree in Electronic Engineering in 1977. Since joining ADI in 1977, he has directly or indirectly contributed more than 30 data converter products and has 13 patents. He was appointed ADI researcher in 1995. In 2009, he transformed from a full-time job and continued to serve as an ADI consultant as an honorary researcher, writing for the “active learning program”. In 2016, he was appointed resident engineer of RPI ECSE department.

Antonio miclaus is now a system application engineer of ADI company, engaged in ADI teaching projects, and works for circuits from the lab ®、 QA Automation and process management develop embedded software. In february2017, he joined ADI in Cluj napoka, Romania. He is currently a master of science student in the master of software engineering program of beibisboyer University, and holds a Bachelor of Science Degree in electronic and telecommunication engineering from the University of science and technology of kluze napoka.

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