I started self-learning about transistors and used Multisim to design a simple voltage amplification circuit.
I'm using this document to record my learning process.

It includes the following content:

I found a typical circuit online and drew it in Multisim.
Learned to use the probes and oscilloscope provided by Multisim.
Looked up transistor parameters online.
Explored the three working states of a transistor.
Summarized the design steps for a transistor voltage amplification circuit.
What determines the amplification factor of the circuit?
What is the role of capacitors in the circuit?

Start from a circuit

I found a single-transistor voltage amplification circuit online using a 2N3904 transistor. I don't fully understand the circuit's working principle yet, but I recreated it in Multisim first. The diagram is shown below.

I connected the input of the circuit to a signal generator (set as a sine signal with a frequency of 1kHz and an amplitude of 10mV). Then, I connected Channel 1 of an oscilloscope to the output of the circuit. My father told me that to stabilize the output signal, a load resistor should be connected between the output and ground. This resistor should not be too small, so a 10kΩ resistor was used. After clicking "Run" to start the simulation, the output signal observed from the oscilloscope was a sine signal with a frequency of 1kHz and an amplitude of 800mV. Therefore, this circuit amplified the voltage of the input signal by 80 times.

The circuit is already working, but I still don't understand its principle. I need to start understanding from the very beginning.

Understand the triode

We can imagine a triode as a special "current faucet" with three small terminals called the collector, emitter, and base. The collector is like the "water inlet" connected to the positive terminal of a battery, the emitter is the "water outlet" connected to the negative terminal, and the base is like the "faucet handle"—but this handle is magical, as it only requires a tiny amount of "force" (a small current) to control the water flow.

When there is no current at the base, it's like the handle isn't turned, and the channel between the inlet and outlet is closed, with almost no current flowing out (this is called the cutoff state). If a small current is applied to the base, similar to gently turning the handle, the channel opens slightly, and a much larger current will flow from the inlet (collector) to the outlet (emitter). The size of the current is proportional to the "force" applied to the handle (base current). For example, if the base uses 1 mA of current to "turn" the handle, the outlet might release 100 mA of current (this is the amplification state). If the base current is too large, the channel will open completely, and the current will no longer increase (this is called the saturation state. Imagine when a faucet is fully open, and the water flow can't increase further).

The Three Electrodes of an NPN Triode

Emitter (E): The "outlet" for current, with an arrow indicating the current direction.
Base (B): The tiny "control terminal", where a small current can manipulate the overall current flow.
Collector (C): The "inlet" for current, connected to the power supply and electrical load.

The Three Operating States of a Triode

Cutoff State (Switch Off):
When there is no base current (or the current is too small), the emitter and collector are disconnected, similar to a faucet fully closed with no water flow.
Amplification State (Small Current Amplified):
When a small base current (e.g., 1 mA) is applied, the collector will output a proportionally larger current (e.g., 100 mA, with an amplification factor β=100), analogous to using a small force on a lever to lift a heavy object.
Saturation State (Switch Fully On):
When the base current is sufficiently large, the collector current reaches its maximum value and no longer increases with further base current, just like a faucet fully opened where the water flow cannot increase anymore.

Triode 2N3904

Triode models produced in the United States often start with "2N", where "N" represents the registration mark of the Electronic Industries Association (EIA), and the subsequent numbers are registration serial numbers.

2N2222: An NPN general-purpose triode with a collector current up to 0.8A and a collector-emitter voltage of 40V. It is commonly used in general amplification, switching circuits, etc., with common packages like TO-18. Its improved version is 2N2222A35.
2N3904: An NPN silicon high-frequency and small-signal triode (main parameters introduced earlier), suitable for general amplification circuits, etc., with common packages like TO-92235.
2N3906: A PNP silicon high-frequency small-signal triode, corresponding to 2N3904, commonly used in general amplification circuits with similar package forms (e.g., TO-92)235.
2N4401: An NPN silicon high-frequency low-power triode applicable to switching circuits, etc., with a typical collector-emitter voltage of 40V and collector current of about 500mA5.
2N5401: A PNP small-signal transistor with a collector-emitter voltage of 150V and collector current of about 0.6A, commonly packaged in TO-92235.
2N5551: An NPN silicon high-frequency low-power triode for switching circuits, etc., with a maximum collector-emitter voltage of 160V and continuous collector current of 0.6A, mostly using TO-92 packaging123.
2N3055: An NPN silicon high-frequency high-power triode with a collector current up to 15A and collector-emitter voltage of 60V, commonly used in audio amplifiers, switching power supplies, etc. It generally adopts larger packages (e.g., TO-3) for heat dissipation.

Parameters of 2N3904

Voltage Parameters:

·Collector-Emitter Voltage (Vceo): 40V

·Base-Emitter Voltage (Vbe): 6V

·Collector-Base Voltage (Vcb): 60V

Current Parameters:

·Maximum Collector Current (Ic): 200mA

·Base Current (Ib) is generally limited within a range, such as 5mA (may vary by application).

·DC Current Gain (hFE): Minimum 100 (@150mA, 10V), maximum 300, typically ranging from 100 to 400^134.

Power Parameters:

·Maximum Power Dissipation: Generally 625mW (slightly different for different packages), with some sources indicating 800mW^134.

Frequency Parameters:

·Transition Frequency (Ft): Typical value is 300MHz, while some sources show different values like 100MHz (may vary due to testing conditions, etc.)

Package Types:

·Common packages include TO-92, TO-39-3, TO-205AD, TO-226-3, etc..

Temperature Range:

·Operating and storage junction temperature range is generally -55°C to +150°C.

Design Ideas and Steps for Amplifier Circuits

Step 1: Determine the optimal amplification operating point Q. Determine the Quiescent Current (IC-Q) and Voltage (VCE-Q)

IC-Q: Typically set as 1/3 to 1/2 of the triode's maximum ICM to balance amplification capability and power consumption. For 2N3906 with ICM = 200mA, IC-Q can be set to 60mA.
VCE-Q: Generally taken as approximately Vcc/2 to ensure the output signal has sufficient dynamic range.

Step 2: VCC is set to 15V, as this is a common output voltage for ordinary experimental power supplies, making it convenient for conducting real-world experiments.

Step 3: VB is set to 5V, which is VCC/3. This is an empirical approach to minimize the impact of power supply fluctuations to the greatest extent.

Step 4: Determine R1 and R2. Since Ib is very small (60mA/100 ≈ 0.6mA), it can be approximated as an "open circuit." Therefore, R1 can be set to 2kΩ and R2 to 1kΩ to achieve VB = VCC/3.

Step 5: Determine the emitter resistor R4. R4=IC−QVB−0.7V=60mA5V−0.7V=71Ω.

Step 6: Determine the collector resistor R3. R3=IC−QVCC−VCE−Q−4.3V=60mA15V−7.5V−4.3V=53Ω

After completing the calculations, design the circuit and perform a simulation. Using voltage and current probes to measure each node shows values approximately matching the setting values. The discrepancies arise from the fact that although the base current (Ib) is small, it still affects the base voltage (VB). Additionally, the current gain (hFE) of the 2N3904 in the simulator does not exactly match the assumed value of 100. However, these deviations do not significantly impact the operational state of the transistor.

Currently, the VCE of the triode is approximately 8V relative to the 15V power supply, indicating it is in the amplification region. If VCE reaches 15V, the voltages across R3 and R4 become zero, with no current flowing - meaning the "valve" is shut off, and the triode enters the cutoff region. Conversely, if VCE is very low (e.g., less than 1V), the "valve" is fully open, offering no resistance to the "current flow," placing the triode in the saturation region.

Key takeaway: Measuring VCE allows you to determine the triode's operating state. (This is a crucial concept - memorize it!)

The Principle of Signal Amplification

The Relationship between Output Signal ΔVc and Input Signal ΔVb

ΔIc = ΔIe = ΔVb/R4

ΔVc = ΔIc x R3

ΔVc = ΔVb x (R3/R4)

Then, according to the values of R4 and R3 in the diagram, the output signal ΔVc is smaller than the input signal ΔVb. Doesn't this mean it's not amplifying but reducing the signal?

Let's run a simulation. As expected, the amplitude of the output signal is 4mV, which is smaller than the input signal! What should we do about this?

Now we know that Vout = Vin × (R3/R4), but R3 and R4 determine the triode's Q point (operating state). How can we make Vout > Vin to achieve signal amplification without changing their values? This is where C2 comes into play. By connecting a capacitor in parallel with R4, which has the function of "blocking DC and passing AC", the AC signal can pass through, effectively paralleling a small resistance with R4 and reducing the equivalent resistance of R4. When simulating with an 82μF capacitor connected in parallel, with Vin still at 5mV, Vout becomes 120mV, achieving a 24-fold amplification.

If we replace this capacitor with a 1000μF one, the Vout becomes 300mV, meaning the Vin is amplified by 60 times. Therefore, the larger the capacitor, the smaller the impedance it offers to the alternating current signal.

The Relationship between Capacitive Reactance, Signal Frequency, and Capacitance

Capacitive reactance (XC) represents the opposition of a capacitor to alternating current, defined by the formula:XC=1/2πfC

XC: Capacitive reactance (unit: ohm, Ω)
f: Signal frequency (unit: hertz, Hz)
C: Capacitance (unit: farad, F)

Final

The power consumption of the transistor 2N3904 in this amplifier circuit is: Vce Ic = 8V 60mA = 480mW.

Learning to Design Voltage Amplifier Circuit Using One Triode