This version (24 Jul 2017 16:23) was *approved* by amiclaus.The Previously approved version (27 Mar 2017 17:04) is available.

The objective of this lab activity is to study the characteristics of tuned amplifiers stages as covered in Chapter sections 16.4 and 16.5 of the on-line Introduction to Electronics text.

Many communications system requirements exceed the high-frequency limits of op-amps. In cases such as these, discrete tuned amplifiers are often used. Discrete amplifiers are typically tuned using LC (parallel inductor-capacitor) resonant circuits in place of the collector (or drain) resistors. One such circuit is shown in figure 1.

Figure 1 Common emitter amplifier with resonate output load

The parallel LC (resonate tank) circuit determines the frequency response of the amplifier. There is a frequency at which X_{L} = X_{C}. This frequency, called the resonant frequency F_{R}, is calculated using:

As we learned in the lab on inductor self resonance it is important to take this built-in capacitance into account when designing tuned amplifiers. In an ideal resonant circuit, inductor current lags the capacitor current by 180º and the net circuit current is zero. As a result: The impedance of a parallel resonant circuit is extremely high at F_{R}. The common emitter amplifier voltage gain reaches its maximum value when the collector load impedance is maximum i.e. operated at F_{R}.

When the input frequency (F_{IN}) is lower than F_{R}, the circuit impedance decreases from its maximum value, and is inductive. When F_{IN} is higher than F_{R}, the circuit impedance drops again, but is capacitive. When operated at F_{R}, the impedance of the tank circuit reaches its maximum value. As a result, the gain of the tuned common emitter amplifier is also at its maximum value.

Build a simulation schematic of the tuned amplifier as shown in figure 1. Calculate values for bias resistors R_{1} and R_{2} such that with emitter resistor R_{3}set to 100 Ω, the collector current in NPN transistor Q_{1} is approximately 5 mA. Assume the circuit is powered from a +10V power supply. Be sure to keep the sum of R_{1} and R_{2} ( total resistance ) as high as practical to maintain the input impedance of the amplifier stage as high as possible. Set input and output AC coupling capacitors C_{2} and C_{3} to 0.1 uF. Calculate a value for C_{1} such that the resonate frequency, with L_{1} set equal to 100 uH, will be close to 500 KHz. 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 simulation schematics for the circuits shown in figures 3 and 4 as well.

ADALM2000 Active Learning Module

Solder-less breadboard, and jumper wire kit

1 - 2N3904 NPN transistor

1 - 100 uH inductor (Various other value inductors)

2 - 0.1 uF capacitors ( marked 104 )

1 - 100 Ω resistor

Other resistor and capacitors as needed

Build the circuit shown in figure 2 on your solder-less breadboard. Based on your pre lab simulations pick values for bias resistors R_{1} and R_{2} from your parts kit such that with the 100 Ω emitter resistor, R_{3}, the collector current in NPN transistor Q_{1} is between 5 mA and 10 mA. Assume the circuit is powered from the +5V and -5V power supplies ( 10 V total ). Be sure to keep the sum of R_{1} and R_{2} ( total resistance ) as high as practical to maintain the input impedance of the amplifier stage as high as possible. Again based on your simulations, calculate a value for C_{1} such that the resonate frequency with the 100 uH L_{1} will be close to 500 KHz. Pick a standard capacitor value from the ones supplied in your parts kit or combine two in series or parallel to come as close as possible to your calculated value. Calculate a new resonate frequency based on the final value you ended up with for C_{1}. You may want to include the effect of the parasitic winding capacitance you might have measured in the lab on inductor self resonance.

The peak gain of this tuned amplifier can be very high. We will need to slightly attenuate the output signal of AWG1 by picking a value for R_{S} that is 2 to 3 times larger than the parallel combination of R_{1} and R_{2} ( the input resistance of the amplifier). The value of the output load, R_{L}, also determines the amplifier maximum gain. For the initial measurements leave R_{L} out of the circuit. The approximate 1 MegΩ input resistance of the scope channel will serve as R_{L}.

Figure 2 Common emitter tuned amplifier

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 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.

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 500 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.

Now add the load resistor, R_{L}, in the circuit. Start with 100 KΩ and run a new sweep. Note the maximum gain and the frequency. Compare to the result you got with just the scope input as the load. Try successively lower values for R_{L} such as 10 KΩ and 1 KΩ etc. Note and compare your measurements.

What is the 3 dB bandwidth of the amplifier without R_{L}?

What is the 3 dB bandwidth with R_{L} equal to 100 KΩ, 10 KΩ and 1 KΩ?

What is the effect of connecting a capacitor in parallel ( across ) with emitter resistor R_{3}?

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?

Frequency Multipliers or harmonic generators are a special class of amplifiers that are biased at 3 to 10 times below normal cutoff bias. They are used to generate an output frequency that is a multiple (harmonic) of a lower input frequency.

The tuned amplifier circuit of figure 2 can operate as a frequency multiplier. If an input signal, such as a square wave or pulse that contains a large enough harmonic, has a frequency of say 167 KHz, which is 1/3 of the 500 KHz resonant frequency of the output tank, the output signal would contain mostly 500 KHz where the gain is highest, or three times the input frequency. The fundamental frequency and other harmonics of the input will be greatly reduced by the tuned nature of the circuit. The fifth harmonic (frequency quintupler) is normally as high in multiplication as is practical, because harmonics of an input signal higher than the fifth are generally very weak, and the multiplied output diminishes to a very weak signal.

Calculate new values for the input bias resistor divider R_{1}, R_{2} such that transistor Q_{1} is nominally just cut off ( I_{C} = 0 ) with no input signal applied. Sine waves generally do not contain any harmonics so set AWG1 to produce a square wave signal at 1/3 the resonate frequency you measured in the earlier tests. To generate large harmonics set the symmetry to 20% ( pulse high for 20% of the period). You will need to increase the amplitude of the input pulse to greater than 2 V or remove the input attenuation source resistor R_{S}.

Frequency multipliers are operated by the pulses of collector current produced by a class C amplifier. Although the collector current flows in pulses, the alternating collector voltage is sinusoidal because of the action of the tank circuit. Use one of the scope channels to monitor the collector current pulses by measuring the voltage across emitter resistor R_{3}.

The following circuit, shown in figure 3 is a more versatile tuned amplifier stage using an NPN differential pair with an LC resonate output load.

ADALM2000 Active Learning Module

Solder-less breadboard, and jumper wire kit

1 - 2N3904 NPN transistor

1 - SSM2212 NPN matched transistor pair

1 - 100 uH inductor (Various other value inductors)

2 - 0.1 uF capacitors ( marked 104 )

1 - 100 Ω resistor

2 - 1 KΩ resistors

1 - 2.2 KΩ resistor

Other resistor and capacitors as needed

Build the circuit shown in figure 3 on your solder-less breadboard. Use the SSM2212 matched transistor pair for Q_{1} and Q_{2}. Pick values for bias resistors R_{1} and R_{2} from your parts kit such that with the 100 Ω emitter resistor, R_{3}, the collector current in NPN transistor Q_{3} is between 5 mA and 10 mA. Note in this case the R_{1}, R_{2} resistor divider is powered from ground and -5V power supply. Use the same combination of L_{1} and C_{1} as in the previous amplifier stage.

Figure 3 Differential amplifier stage with single ended resonate output load

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.

As in the first experiment, run a single frequency sweep. Once you have determined that the maximum gain of the amplifier occurs near 500 KHz then you can reduce the frequency sweep range to start at 100 KHz and stop at 1 MHz. Be sure to export the data to a .csv file for further analysis in either Excel or Matlab.

As before add the load resistor, R_{L}, in the circuit. Start with 100 KΩ and run a new sweep. Note the maximum gain and the frequency. Compare to the result you got with just the scope input as the load. Try successively lower values for R_{L} such as 10 KΩ and 1 KΩ etc. Note and compare your measurements. They should be very similar to the results you got in the first experiment.

Amplitude modulation may be applied to the output frequency by capacitor coupling a modulating (audio frequency) signal from AWG 2 to either the base or emitter of current source transistor Q_{3}.

It is sometimes desirable to include a simple active high pass filter to the input of the single transistor tuned amplifier stage. The filter circuit shown in figure 4 provides a two pole filter with unity gain. This filter is convenient to place in a larger circuit because it contains few components and does not occupy much space.

The active high pass transistor circuit is quite straightforward, using just a total of four resistors, two capacitors and the same single transistor. The operating conditions for the transistor are set up in the normal way. As in figure 1, R_{1} and R_{2} are used to set up the bias point for the base of the transistor. The resistor R_{3} is the emitter resistor and sets the current for the transistor.

The filter components are included in negative feedback from the emitter of the transistor to the input. The components that form the active filter network consist of C_{2}, C_{3}, R_{4} and the combination of R_{1} and R_{2} in parallel, assuming that the input resistance to the base of the transistor is very high and can be ignored.

This is for values where the effect of the transistor itself within the high pass filter circuit can be ignored, i.e.:

Where:

β = the forward current gain of the transistor

F_{o} = the cut-off frequency of the high pass filter

π = equal to 3.14159

The equations for determining the component values provide a Butterworth response, which provides maximum flatness within the pass-band at the expense of achieving the ultimate roll off as quickly as possible. This has been chosen because this form of filter suits most applications and the mathematics works out easily.

ADALM2000 Active Learning Module

Solder-less breadboard, and jumper wire kit

1 - 2N3904 NPN transistor

1 - 100 uH inductor (Various other value inductors)

1 - 0.1 uF capacitors ( marked 104 )

1 - 100 Ω resistor

Other resistor and capacitors as needed

Build the circuit shown in figure 4 on your solder-less breadboard. Use the same values for bias resistors R_{1} and R_{2} that you used in experiment 1 (figure 2). Use the same combination of L_{1} and C_{1} as in the previous amplifier stage. Using the formula above for the high pass cut off frequency F_{O}, calculate values for C_{2}, C_{3} and R_{4} that results in a frequency more than two octaves below the resonance frequency of L_{1} and C_{1}. For example if F_{R} is equal to 500 KHz then base your calculations on F_{O} equal to 125 KHz.

Figure 4 Adding a 2 pole high pass input filter to the tuned amplifier

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.

As in the first experiment, run a single frequency sweep with scope channel 2 connected through coupling capacitor C_{4} to the collector of Q_{1}. To measure the response of the high pass input filter, connect scope channel 2 through coupling capacitor C_{4} to the base of Q_{1}. Be sure to export the data to a .csv file for further analysis in either Excel or Matlab. Compare the response curves with what you measured for the circuit in figure 2. Try different combinations of values for C_{2}, C_{3} and R_{4} to see how the frequency response changes.

**For Further Reading:**

http://en.wikipedia.org/wiki/LC_circuit

http://en.wikipedia.org/wiki/Q_factor

http://en.wikipedia.org/wiki/Tuned_radio_frequency_receiver

**Return to Lab Activity Table of Contents.**

university/courses/electronics/comms-lab-tuned-amplifiers-1.txt · Last modified: 24 Jul 2017 16:22 by amiclaus