# Analog Devices Wiki

This version (24 Jul 2017 16:26) was approved by amiclaus.The Previously approved version (27 Mar 2017 17:06) is available.

# Activity: Tuned Amplifier Stages, part II

## Objective:

The objective of this lab activity is to continue the study of tuned amplifiers stages that was started in this earlier set of activities.

## Background:

As we learned in the previous set of activities, second order LC tank circuits are commonly used as the tuned element in amplifier stages. The simple parallel LC tank, as shown in figure 1, can produce voltage gain at the expense of current to drive a resistive load. A buffer amplifier such as an emitter follower can supply the required current ( or power ) gain to drive a load.

Figure 1 parallel resonate LC tank circuit

The second coupling capacitor, C2, must be included in the calculation of the resonate frequency. The following formula will give us the resonate frequency for the circuit in figure 1:

### Pre Lab Simulations

Build a simulation schematic of the tuned emitter follower amplifier as shown in figure 1. Calculate a value for emitter resistor RL such that the current in NPN transistor Q1 is approximately 5 mA. Assume the circuit is powered from +/- 5V power supplies (+10V total ). Hint: the DC voltage at the base of Q1 is set by the DC path through L1 to ground. Calculate a value for C1 and C2 such that the resonate frequency, with L1 set equal to 100 uH, will be close to 350 KHz. Generally, C1 and C2 are of equal value. Perform a small signal AC sweep of the input and plot the amplitude and phase seen at the output. Save these results to compare with the measurements you take on the actual circuit and to include with your lab report. You may also want to create a simulation schematic for the circuit shown in figures 3 as well.

### Materials:

Solder-less breadboard, and jumper wire kit
1 - 2N3904 NPN transistor
1 - 100 uH inductor (Various other value inductors)
2 - 1.0 nF capacitors ( marked 102 )
2 - 1 KΩ resistors
1 - 2.2 KΩ resistor
Other resistor and capacitors as needed

### Directions:

Build the circuit shown in figure 2 on your solder-less breadboard. Use a 100 uH inductor for L1 and 1.0 nF capacitors for C1 and C2. The peak gain of this tuned amplifier can be very high at the resonate frequency. We will need to slightly attenuate the output signal of AWG1 using resistor divider RS and R1.

Figure 2 Emitter follower tuned amplifier

### Hardware Setup:

The green squares indicate where to connect the ADALM2000 module AWG, scope channels and power supplies. Be sure to turn on the power supplies only after you double check your wiring.

Open the power supply control window to turn on and off the +5 and -5 volt power supplies. Open the network analyzer software instrument from the main Scopy window. Configure the sweep to start at 10 KHz and stop at 10 MHz. Set the Amplitude to 200 mV and the Offset to zero volts. Under the Bode scale set the magnitude top to 50 dB and range to 80 dB. Set the phase top to 180º and range to 360º. Under scope channels click on use channel 1 as reference. Set the number of steps to 500.

### Procedure:

Turn on the power supplies and run a single frequency sweep. You should see amplitude and phase vs frequency plots that look very similar to your simulation results. Once you have determined that the maximum gain of the amplifier occurs near 350 KHz then you can reduce the frequency sweep range to start at 100 KHz and stop at 1 MHz. Be sure to export all the frequency sweep data to a .csv file for further analysis in either Excel or Matlab.

### Questions:

What is the output impedance seen at the emitter of Q1? Compare this to the output impedance seen at the collector of the common emitter tuned amplifier we explored in the previous lab activity. Using the scope and function generator software instruments ( in the time domain ) what is the maximum peek to peek voltage swing possible at the output of the circuit? Be sure to measure it at the resonance frequency. What limits the positive and negative peak voltages? Can it be larger than the power supply voltage and why or why not?

## Tuned amplifier with Quadrature Outputs

If we add a second conventional emitter follower stage as a non-tuned parallel path we can will have an amplifier with two outputs that will have exactly a 90º phase difference between them, at the resonant frequency. By adding a resistor in parallel with the resonate tank, L1, C1, we can lower the gain at resonance to unity ( 0 dB ) such that the gain from the input to the emitter of Q1 will be the same as the non-tuned gain unity gain of the conventional emitter follower stage, Q2.

1 - 2N3904 NPN transistor
2 - 470 Ω resistors
1 - 1 KΩ resistor

### Directions:

Modify the circuit on your solder-less breadboard to add the second emitter follower stage, Q2, as shown in figure 3. Be sure to turn off the power supplies and stop the AWG before making any changes to your circuit.

The exact value for R1, such that the gain is reduced to unity, may vary from the 470 Ω suggested in the figure. You can experiment with different values to obtain the proper amount of gain to match the amplitude seen at the emitter of Q2.

Figure 3 Amplifier with quadrature outputs

### Hardware Setup:

The green squares indicate where to connect the ADALM2000 module AWG, scope channels and power supplies. Be sure to turn on the power supplies only after you double check your wiring.

### Procedure:

Set the AWG amplitude in the Network Analyzer to 2.0 V, because we have reduced the gain by adding R1. Turn on the power supplies and run a single frequency sweep. You should see amplitude and phase vs frequency plots that look very similar to your simulation results. Be sure to export all the frequency sweep data to a .csv file for further analysis in either Excel or Matlab.

Using the scope and function generator software instruments ( in the time domain ) set the AWG frequency to the resonate frequency with the amplitude set to 2V. Observer the relative amplitude and phase of the two outputs.