7
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Cascode BJT amplifier DC analysis assumption
Initial Answer
I won't perform the solution (yet.) But I can suggest a process for analyzing the circuit when the base currents aren't assumed to be zero.
Let's look at the annotated schematic:
...
- 78.7k
7
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How does one set the Q-point for a JFET cascode amplifier, and what are the correct "Idss" requirements for cascoded JFETs?
Beyond formal classifications
Formalisms simplify and facilitate our work, but they make us not think and apply ready-made patterns. Such is the formal "common-something stage" ...
- 19.5k
6
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Is the main purpose of cascoding is to increase gain in FETs?
It is mainly done to increase the output resistance and or reduce the Miller effect.
An example of increasing output resistance is the cascode current mirror.
An example of Miller effect mitigation ...
- 4,440
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Vintage Oscilloscope Preamp Whitepaper
Perhaps C.R. Battjes "AFTR Class Notes" Tektronix 1977. It describes a cascode "ft doubler" circuit. Referenced in Wideband Amplifiers (Starříč & Margan).
6
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Definition of cascode?
A cascode, loosely speaking, is an arrangement wherein a common-source (or common-emitter) stage feeds into a common-gate or common-base stage, in order to combine the transconductance of the former ...
- 20.6k
5
votes
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Condition for the output and input impedance for cascading electronic circuits
\$R_1\$ and \$R_2\$ form a voltage divider which reduces the voltage at the input of the second stage. The Thevenin equivalent of the first stage combined with the second stage modeled as an input ...
Null♦
- 7,652
4
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Biasing high frequency BJT cascode amplifier
Wideband amplifiers are challenging from the point of view of both the design and practical realization: the cascode stage is no exception, and below I try to give some answer to the question posed.
...
- 3,507
4
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Is the main purpose of cascoding is to increase gain in FETs?
In a common base circuit, the Miller capacitance no longer has any effect on the high frequency gain being rolled off by negative feedback. So the 2nd stage of the cascoded pair is the best you can ...
- 473k
4
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Cascode vs Folded-Cascode
There are two uses of a cascode -- one is to protect a low voltage transistor from a high voltage supply; the other is to increase the gain of the circuit.
In the circuit (a), M1 drives into a low ...
- 20.3k
4
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MOS Cascode - Shielding Property
If the M1 drain was connected directly to Vout, then it has the Vout voltage swing on it.
As Vb is constant, the voltage of M2 source is very nearly constant, with orders of magnitude less voltage ...
- 174k
4
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What is folded cascode?
Here is example of folded cascode amplifier [source: B. Razavi, Analog CMOS Integrated Circuits]:
Typical for folded cascode topology is using different branch for generating small signal currents, ...
- 131
3
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Cascode Current Mirror, Minimum output voltage
Why is the minimum output voltage, on the drain of the Q4, equal to the Vout_min=Vt+2 Vds_sat, why not only 2 Vds_sat?
You're partly right that one could also consider \$2 * V_{ds,sat}\$ but then the ...
- 81.4k
3
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Why cascode transistors contribute little to overall noise in an OTA?
Noise contribution of a device at the output will be multiplied by the gain from the noise source input to the output. Voltage gain for a transistor can be approximated as $$A_v = \frac{Z_d}{Z_s},$$ ...
- 3,796
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Is the main purpose of cascoding is to increase gain in FETs?
I have created a cascode vs. single transistor amplifier comparison circuit in this free on-line simulator:
https://www.systemvision.com/design/compare-cascode-vs-single-mosfet-amplifier
In the ...
3
votes
Is the main purpose of cascoding is to increase gain in FETs?
Cascodes are:
used as the gain elements in amp stages when the Miller effect is an issue.
used in current sources and as non-linear loads where the output resistance of a single BJT isn't enough.
...
- 78.7k
3
votes
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How does JFET cascode amplifier reduce input capacitance?
Let's say I disregarded the upper JFET and applied a fixed DC bias of 10 volts to the lower JFET's drain as per this diagram: -
Although it isn't a voltage amplifier any more, can you see that that ...
- 473k
3
votes
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Advice to calculate the transfer function of circuits
Hint: -
Just use your eyes and simplify. Here's a big start: -
Now it's your turn - take this diagram and simplify each stage to just its gain values.
No need to use KCL if you understand how op-amps ...
- 473k
3
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Why isn't my current mirror reflecting 500uA on each side?
You've run out of headroom. Note M1's operating point:
Vds is significantly smaller than Vgs in magnitude, suggesting that the transistor has entered triode, its small signal output impedance has ...
- 20.6k
2
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Increase low-frequency gain of a cascode amplifier
If I were to build this circuit using an aluminum electrolytic
capacitor for C2, will I run into ESR problems at high frequencies?
Not only ESR but ESL (inductance) will cause a resonant frequency ...
- 473k
2
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Cascode Amplifier Gain
Since gmb2 is usually ignored when the substrate connection is properly grounded, I'll ignore it and make thing slightly simpler. So gmb2 = 0
Then looking "up" into the source of M2 I see:
$$1/(gm_2)...
- 81.4k
2
votes
MOSFET Cascode: intuitive explanation
To answer this question, I will try to build the above circuit which you have step by step and explain intuitively at each step. I will take N-MOSFET for simplicity.
If we take a simple transistor (...
- 446
2
votes
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Simulation result of bjt cascode amplifier
As stated in my comment above, it seems to me that the problem of your circuit is that, at the fundamental frequency of the test input signal \$V_5\$, it behaves as a differentiator. I suggest to ...
- 3,507
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Mid-band magnitude gain of a Cascode Amplifier
Using this small-signal diagram
We clearly see a voltage divider at the input.
Therefore
$$\frac{V_{B1}}{V_S} = \frac{R_1||R_2||(\beta_1 +1)r_{e1}}{R_S + R_1||R_2||(\beta_1 +1)r_{e1}}$$
Next, ...
- 15.7k
2
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Questions about cascode current mirror with MOSFETs
The point of a current mirror is the programming of Iout by the gate voltage from an identical device. If Iin flows through M3 it develops a gate voltage which is in common with the gate voltage on M4....
- 1,504
2
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Finding the gain expression for this CMOS operational transconductance amplifier
As the image mentions, the amplifier you've shown is a hybrid telescopic and folded cascode amplifier. Essentially, it is a telescopic cascode amplifier and a folded-cascode amplifier combined so that ...
- 607
2
votes
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Role of input capacitance for cascode-input op-amp
In my experience, I have never seen anyone place an explicit capacitor at the input of an op-amp for stabilization purposes. As DKNguyen commented, this is more of a representative model of the ...
- 4,708
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votes
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Cascode amplifier does not provide gain properly
A 53 μF tank capacitor is more suitable for sub-audio filtering. It's completely inappropriate for an amplifier operating at 250 kHz.
Something like 4.05 nF and 100 μH is more like the values you need ...
- 473k
2
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Folded Cascode op-amp bias point
There are a couple of points we need to consider.
A folded cascode topology is generally used to get good input and output common modes. If bias is connected to the tail of the diff pair, the output ...
- 4,390
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votes
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How a cascode GaN can have a low Rds(on)?
The silicon MOSFET does not need to block much voltage so it can be cheaply made to have an Rds(on) that does not add much to that of the GaN HEMT. For example, a 0.05 ohm compound device might be 0....
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