BJT circuit with cascaded stages

In summary, the conversation discusses a cascade divided into three sections: an emitter follower on the left, an unbiassed BJT in the center, and another unbiassed BJT on the right. The total voltage and resistors are calculated using equations involving the current and resistance values. The use of pull-down resistors is suggested to help set the bias points. The possibility of difficulty achieving the required gain in real-world applications is acknowledged.
  • #1
ac7597
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Homework Statement
Use 2N3904 NPN transistors to design an amplifier which will meet the electrical specifications. Several (three) cascaded stages will be necessary.
Amplifier Specifications at 1kHz:
Zin=47k ohms minimum
Zout=4.7k ohms maximum
Overall voltage gain(no load) | Avnl |= 1000 minimum
Output voltage swing capability: at least 5V p-p without visible distortion Power supply voltage available is Vcc=15V DC

Show schematic diagram from Multisim, including reference designators and parts values. The DC voltages from a bias simulation should appear on the schematic. Do a DC bias simulation of all nodes, and then use “Place Text” to enter the values right onto the schematic at each node.

A Multisim plot of the input and output voltages when the input to the amplifier is an AC source at 1kHz with a peak-to-peak amplitude of 5mV. The output voltage should be a clean sine wave with no visible clipping or distortion. The output voltage amplitude should be clearly shown as being at least 5V peak-to-peak, thereby proving that your gain is at least the required value of |1000|.
To start:
For the initial 2N3904 parameters values use beta=150, Vbe=0.7V, ro=50k ohms.
The resistor values should be standard values that can be brought.
Relevant Equations
AV total= AV1 * AV2 * AV3
This cascade is divided into 3 sections thus:
the leftmost section is an emitter follower :
Ib1 = (15V-0.7V)/ ( R1 + 151R2)
IE1= 151*Ib1
re1= 26mV /(IE1)
Zb1= 150( re1+ R2)
AV1= (-150*R1)/ (R1+Zb1)

the center section is an unbiassed BJT :
Ib2 = (15V-0.7V)/ ( R3 + 151(R4+Re2a) )
IE2= 151*Ib2
re2= 26mV /(IE2)
Zb2= 150( re2+ Re2a)
AV2= (-150*R3)/ (R3+Zb2)

the rightmost section is an unbiassed BJT :
Ib3 = (15V-0.7V)/ ( Rb3 + 151(Rc3+Re3a) )
IE3= 151*Ib3
re3= 26mV /(IE3)
Zb3= 150( re3+ Re3a)
AV3= (-150*Rb3)/ (Rb3+Zb3)

AV total= AV1 * AV2 * AV3 >=1000

output voltage= AV total * (0.005V) >=5V

Is this layout correct? How do I proceed further to find the resistors?
 

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  • #2
What do you mean by "unbiassed BJT"? IMO you need a pullodown resistor on each base to help you set the bias points.

Can you make that correction and show us your simulation results? Thanks. :smile:
 
  • #3
I must admit that I am too lazy for finding the meaning (and checking the correctness) for symbols like Zb1 and Re2a and...(without a minimum of explanation).
 
  • #4
berkeman said:
IMO you need a pullodown resistor on each base to help you set the bias points.
I think the operative word there is help.
With no spread of the characteristics, Beta, VBE or resistor tolerance, I see that biasing as valid. I've actually seen a simpler version of it in some consumer products (many years ago anyhow, haven't looked recently).

Real-World, getting the required gain with that circuit could be... interesting; but certainly worth the effort.
 

1. What is a BJT circuit with cascaded stages?

A BJT (Bipolar Junction Transistor) circuit with cascaded stages is a type of electronic circuit that consists of multiple stages of BJT amplifiers connected in a series. Each stage amplifies the input signal and passes it on to the next stage, resulting in greater overall amplification.

2. How does a BJT circuit with cascaded stages work?

A BJT circuit with cascaded stages works by using the amplification capabilities of BJTs to amplify a weak input signal. The input signal is fed into the first stage, where it is amplified and passed on to the next stage. This process repeats for each stage, resulting in a larger output signal.

3. What are the advantages of using a BJT circuit with cascaded stages?

There are several advantages to using a BJT circuit with cascaded stages. Firstly, it allows for greater overall amplification of the input signal. Additionally, it offers better noise performance and higher bandwidth compared to a single stage amplifier. It also allows for more precise control over the amplification process.

4. What are the limitations of a BJT circuit with cascaded stages?

One limitation of using a BJT circuit with cascaded stages is that it can introduce unwanted phase shifts in the output signal, which can affect the overall performance of the circuit. It also requires careful design and tuning of each stage to ensure proper functioning. Furthermore, it can be more complex and expensive to implement compared to a single stage amplifier.

5. What are some common applications of a BJT circuit with cascaded stages?

A BJT circuit with cascaded stages is commonly used in audio amplifiers, radio frequency (RF) amplifiers, and communication systems. It is also used in medical equipment, instrumentation, and other electronic devices that require high amplification of signals.

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