8.31.2021 -- The purpose of this lab is to explore different types of amplifiersand how they vary in voltage gain. In lab 2, students focused on building different variations of common-emitter amplifiers. Some of the amplifiers built in this lab include common-collector amps, push-pull amps, op-amps, and variations of two-stage amplifers.
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3.1 Buffer Amplifier
A buffer amplifier is designed to deliver signals without much distortion or noise interference to the second stage of an amplifier circuit. This is typically implemented with BJT as a common-collecter (CC) amplifier. CC amps have an input signal at the base terminal and measure the output signal at the emitter terminal. The voltage gain for the CC amp is around 1 V/V, but there is a relatively large current gain. The common-collector amp is also known as emitter-follower amplifier.
3.2 LTspice: Two-Stage BJT Amplifier
The answers listed below questions in the lab manual. Questions 1-3 refer to the circuit in Figure 3.1. Questions 4-7 refer to Figure 3.2.
Figure 3.1 Common Collector (CC) Amplifier
Figure 3.2 Two-Stage CE-CC Amplifier
Q(2.16076 V, 127.718mA)
Gain (RL = 10 Ω) = 0.8985 V/V
When RE is increased to 100 Ω, the Vout signal experiences clipping at the troughs of the wave and the Vout wave’s maximum amplitude decreases slightly.
refer to Figure 3.2 for the two-stage CE-CC amplifier
PQ=VCC × IC = −1.23176 W
Gain = Vo ÷ Vin = 53.64 V/V
When the input signal’s amplitude changes from 10mV to 20mV, Vout shows clipping at the bottom of the signal around -1.1V.
3.3 Build and Test the Two-Stage Amplifier
Before constructing the two-stage amplifier, the student needs to set up the speaker. The speaker used in the lab is a "simple device consisting of a paper or plastic cone affixed to a voice coil (an electromagnet) suspended in a magnetic field" (definition from Lab 3 Manual). This speaker will act as the load impedance (about 8 ohms) to measure the output signal from. The speaker is also polarized, almost like a diode, so the speaker won't make noise (or have a decent output signal) if it is connected to the circuit backwards. The speaker did not have wires to connect it to the breadboard, so I soldered a red wire to the positive side and a black wire to the negative end of the speaker.
soldered the wires to the speaker, refer to above paragraph for how that was implemented
Vin = 2 Vpp, 1 kHz 🠒 Vin didn't cahnged much when connected directly to the speaker
CE amplifier Vin = 20 mVpp, 1 kHz 🠒 high-pitched noise from the speaker, comfortable noise level. Vin = 2 Vpp, 1 kHz 🠒 high-pitched and loud noise (enough to startle me a little)
CC amplifier Vin = 2 Vpp 🠒 higher pitch than the results of exercise 3
Two-stage amplifier CE-CC amplifier: see Table 3.1
Vsupply = 18.3 V, R = 100 Ω Pq = Vsupply^2/R = (18.3^2)/100 = 3.3489 W
output amplitude | 987.1 mV |
input amplitude | 1.213 mV |
gain | 813.768 V/V |
Table 3.1 voltage amplitudes and gain for the two-stage amplifier terminated in a 10 Ω load
3.4 The class AB push-pull Amplifier
A CC amplifier is classified as a class A amplifier, where the Q-point is set but there is power dissapated through the transistor regardless if there is an input signal. A class B amplifier used in lab is shown in Figure 3.something. The configuration of the npn and pnp transistors allows the positive and negative cycles of the input signal to be outputed. However, because the the transistors need about 0.7 V to turn on, the output signal is distorted. To supply a sufficent amount of voltage to keep the transistors on, the class AB push-pull amplifier connects diodes at the base of the transistors to solve the distortions issue with the signal.
3.4a LTspice Simulation
refer to Figure 3.3 for constructed circuit
PQ=VCC × ICC = -40.5 mW
refer to Figure 3.4 for constructed circuit
refer to Figure 3.4 for constructed circuit and graph
Pavg = -274.09 mW
refer to Figure 3.5 for constructed circuit
PQ=VCC × ICC = -41.569 mW
refer to Figure 3.5 for constructed circuit and graph
Pavg = -501.33 mW There is more power dissipated in the class B push-pull amplifier with the 2 diodes than that of the amplifier without the diodes.
Figure 3.3 Class AB push-pull amplifier w/o diodes
Figure 3.4 Class AB amplifier - Power dissapated by Vcc+
Figure 3.5 Class AB amplifier w/ diodes
3.4b Build and Test Circuit
PQ = Vsupply × ICC = 883.6 mW
refer to Figure 3.6 below. The measured waves look similar to the simulated results in Figure 3.3. The output wave begins charging and discharging when the input wave reached ±0.7 V, so the output wave isn't a clean sine wave.
PQ = Vsupply × ICC = 960.4 mW
The output signal for the push-pull amplifier with the diodes looks much smoother because the transistors will always have a suffcient amount of voltage to keep them on. This helps with measuring the output as a clean sine wave. See Figure 3.7 for the measured input and output.
The input settings, 2 Vpp at 1 kHz, are sufficient for the speaker to make a noise.
refer to Figure 3.8 for measurements
see Table 3.2 for the gain
At 20 mVpp, the speaker is extremely quiet.
output amplitude | 671.9 mV |
input amplitude | 20.80 mV |
gain | 32.3029 V/V |
Table 3.2 Gain from physical amplifier
Figure 3.6 Class AB push-pull amplifier without diodes, measured result
Figure 3.7 Push-pull amplifier with diodes - measured result
3.5 LM386 Amplifier
The LM386 amplifier is constructed as an integrated circuit on a chip. It is designed for low voltage applications such as audio amplifiers.
(pin out information bullet)
I ended up using 0.1μF and 220μF capacitors for my circuit. See Figure 3.8 for the circuit built.
20 mVpp - loud and somewhat scratchy noise from speaker 100 mVpp - extremely loud noise! sounded a little smoother though
Av = ~50 V/V PQ=Vsupply × ICC = 1.2321 W
Figure 3.8 LM368 amplifier design - 250μF replaced with 220μF and 0.5μF replaced with 0.1 μF
3.6 Op Amp Audio Amplifier
Op Amps have an extremely high gain, high input resistance, and low output resistance, which make it useful for many applications, such as buffer amplifiers and filters. For this lab, the op amp used is the TL071 op amp chip. The pin out diagram for the TL017 is shown in Figure 3.9.
Figure 3.9 Pin out diagram for TL071 op amp
3.6a LTSpice Simulation
LTSpice construction, see Figure 3.10 for original circuit
LTSpice construction, see Figure 3.10 for original circuit
LTSpice construction, see Figure 3.10 for original circuit
LTSpice construction, see Figure 3.10 for original circuit
Av = Vo/Vi = -100 V/V
Av = Vo/Vi = 100 V/V The magnitudes of the gains found are the same: both at 100 V/V. However, the theoretical gain is negative and the measured gain from the simulation is positive. See Figure 3.10
When R3 is 10 Ω, the output wave appears to be extremely clipped, so it almost looks like a square wave. See Figure 3.11 for the output and circuit.
When Vcc- is connected to ground, the maximum peak output voltage is 2V and the output voltage never goes below 0V. This is because Vcc- is connected to ground and not a negative voltage. See Figure 3.12 for the output and circuit.
Av = 1.6750 V / 20 mV = 83.75 V/V
Figure 3.10 Op amp circuit - original
Figure 3.11 Op amp circuit - R3 = 10Ω
Figure 3.12 Op amp circuit - Vcc- is connected to ground
3.6b Build and Test Circuits
Build circuit, original shown in Figure 3.10 above
Av = 256 mV / 25.6 mV = 10 V/V
test the circuit with the speaker!
Refer to Table 3.3 below for results wof other amplifier combinations
| gain (V/V) | PQ (watts) |
CE/CC amp | 19.55 V/V | 0.0892 W |
CE/class AB amp | 23.83 V/V | 0.03424 W |
op amp/class AB amp | 74.13 V/V | 0.0675 W |
LM386 amp | 22.7 V/V | 0.045 W |
Table 3.3 Gain and quiescent PQ various amplifiers
Conclusion
In this lab, we tested different combinations of two-stage amplifiers to figure oiut which one woudl be the best to use for the complete AM radio. I concluded from the LTSpice simulations that the LM386 op amp would be the best amplifier to use because it had the largest gain and it would be very easy to connect. However, I found that the LM386 didn't have the best gain and was a little difficult to use. According to Table 3.3, the op amp/class AB amp combination had the best gain, so hopefully I can use this amplifier in the final AM radio.
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