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A short class with Sarah and her students. The other ways are under More.
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Today, we're going to focus on the common base amplifier. Can anyone tell me what the main features of a common base configuration are?
Isn't it that the input is applied to the emitter and the output is taken from the collector?
Exactly! The common base amplifier has low input impedance and high output impedance, making it quite effective for certain applications. Let's remember this with the acronym LIH—Low Input, High Output!
What's its main application?
Great question! It’s often used in applications requiring current amplification, like RF amplifying. Now, let's see how we calculate the operating point of a transistor in this setup.
Now let's compute the operating point. Suppose we have a voltage supply of 12V and we define two resistors for biasing. How would we begin our calculation?
We should first calculate the voltage across the base! I remember it from my studies—it's the voltage divider formula.
Absolutely! The base voltage can be calculated using the Thevenin equivalent. Does anyone remember how to set that up?
We set it up as V_b = R_A / (R_A + R_B) * V_dd, right?
Right again! What do we get when we plug in our values?
If R_A and R_B are both 100kΩ, then V_b would be 6V!
We've established the DC operating point, which brings us to output swing. What do you think limits the output voltage swing in common base amplifiers?
Could it be the saturation of the transistor?
Exactly! The output swing is constrained by how far the collector voltage can fall before entering saturation. Can anyone calculate an example?
If our output is at 9V and we can tolerate a drop to 5.75V before saturation, that's a 3.25V swing!
Wonderful! It's crucial we understand these limitations to design effective circuits.
Now let’s transition to the common gate amplifier. What unique feature distinguishes it from the common base?
In the common gate configuration, the gate is used as the input terminal.
Right! And like common base, it also has low input impedance. Does anyone recall how we determine its operating point?
It’s through using the currents at the source and threshold voltages!
Well done! Let’s calculate these values together using a numerical approach.
A transistor configuration with low input impedance.
The DC voltage and current level for transistor operation.
The limits of voltage change at the output node.
A configuration used in MOSFET circuits, similar to the common base.
When a transistor is fully conducting, affecting output voltage.
In common base, signal flows, low input high output, that’s how it goes!
Imagine a busy office where people share a common desk; they’re efficient, just like the common base amplifier that maximizes all resources to boost signals.
For output swing, remember: E = Exit (emitter), C = Come back (collector). This is how it flows!
For common gate: CGA - Common Gate Amplifier. Remember what role the Gate plays!
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Common Base Amplifier Configuration
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Operating Point Calculation
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Output Swing Limitations
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Common Gate Amplifier Characteristics
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An amplifier configuration where the base terminal is common to both input and output, providing high frequency response but low input impedance.
The DC voltage and current level at which a transistor operates, determined through biasing.
Parameters like transconductance and output resistance that define how a transistor operates under small signal conditions.
The state of a transistor when it is fully on, causing the collector-emitter voltage to drop near zero.
An amplifier configuration using MOSFETs where the gate terminal is seen as common to input, leading to low input impedance.
This section focuses on common base and common gate amplifiers, analyzing practical circuit biasing arrangements and their numerical examples.
The section discusses common base and common gate amplifiers, providing a detailed approach to the use of practical bias arrangements. It includes numerical examples to derive operating points, small signal parameters, input and output impedances, and current gains, aiming to enhance the understanding of amplifier performance in electronic circuits.
In this section, we delve into common base and common gate amplifiers, specifically focusing on practical bias arrangements that differ from the ideal cases discussed previously. The lecture illustrates how to derive the necessary parameters from these circuits through numerical examples.
The common base amplifier configuration is explored with a clear example involving a voltage divider biasing. By adjusting the values of the resistors involved in the biasing circuit, different voltages at the BJT base can be generated. The section carefully details the calculations for the operating point, beginning with a 12V supply and leading to important conclusions about the biasing impact on emitter current.
Through a step-by-step numerical example, the operating point of the transistor is determined by applying the known values of resistors and voltages. The calculations show how to find the base, collector, and emitter currents, revealing how small-signal parameters like transconductance (g_m) and output resistance (r_o) factor into the circuit's performance.
An important aspect discussed is the output signal swing in both negative and positive directions, showing the limitations based on the DC operating point obtained during the calculations. The negative swing is constrained by the forward bias conditions of the base-collector junction, while the positive swing is subject to the supply voltage constraints.
Following the common base amplifier analysis, the lecture transitions into common gate configurations. While some principles overlap, the complexities of the MOSFET operation in saturation mode come into play. A detailed calculation of the operating point for MOSFET is outlined, similar to the BJT, but in a different context, focusing on the interaction between gate-source voltages and source currents.
The discussion concludes with an exploration of current gain, emphasizing that for both amplifiers, the current gain approximates unity under certain conditions, linking back to the overall performance of the amplifiers in their respective configurations.