Focus on Numerical Examples

In this chunk, we are looking at a unique configuration of a differential amplifier that incorporates both MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) and BJTs (Bipolar Junction Transistors). The integration of these two types of transistors is significant because they have different operational characteristics. The speaker emphasizes that combining them is feasible if fundamental design principles are adhered to. This strategy aims to provide more confidence in circuit design, showcasing versatility in using different transistor types.

This section discusses a modification to the amplifier circuit where a resistor (denoted as R) has been replaced by a different device, likely a transistor that behaves as an ideal current source. An ideal current source should supply a constant current regardless of the voltage across it, though in reality, it has some 'conductance,' which is a measure of how easily current flows through it. The 'inverse of this conductance' is represented as r_o, which indicates its resistance to changes in current flow.

To find the DC current in the amplifier, the speaker highlights the approach of applying Ohm's Law along with the transistor's behavior. The base resistor (R) is connected to a 12V supply and has a specific value (570 kΩ). Using the base-emitter voltage (V_BE) of 0.6V, the base current (I_B) can be calculated as 20 µA. Since the transistor has a beta (β) value of 100, we can derive the collector current (I_C) which results in 2 mA. This calculation is essential to understand how the circuit functions under DC conditions.

In this segment, the speaker compares the left and right branches of the amplifier circuit, noting their symmetry. Because both branches are identical and supplied with the same DC voltage (V_INC), the collector currents in both transistors are equal, set at 1 mA. This equilibrium is crucial for the differential amplifier's operation as it ensures balanced outputs for further analysis.

This part elaborates on the output voltage and its dependency on certain parameters. The operation of the transistors needs to remain in their active region. As long as the input voltage (V_INC) is above a minimum threshold (2 V) and the collector-emitter saturation voltage (V_CE(sat)) is kept to a maximum (0.3 V), the required collector current can be adjusted between 1 mA to 2 mA as needed. These conditions ensure that the transistors work effectively without entering into saturation unexpectedly, which would alter the expected output.

The concluding remarks focus on defining the acceptable range of input common mode voltage (V_INC) in the circuit. The speaker indicates that it can vary from a lower limit of 2.3 V to an upper limit of 9 V. This range is determined by the circuit conditions and is critical for maintaining the amplifier's performance and signal integrity, preventing distortion or degradation of the output signal.

In this section, the formula for calculating common mode gain is provided. The common mode gain is influenced by the transconductance (g_m) and the load resistance (R). The denominator includes a term dependent on how the active devices interact, particularly with their output characteristics, which highlights the complexity of amplifier design in a differential configuration. By manipulating these parameters, one can optimize the amplifier's response to common mode signals, enhancing its performance.

Here, the focus shifts to comparing common mode gain with differential mode gain. The calculated common mode gain is evaluated using previously determined parameters. The numerator represents the output, while the denominator incorporates the interactions of currents and resistance in the design, showcasing how common and differential signals are treated differently in the amplifier. This relationship emphasizes careful circuit design to optimize the performance of both gain modes for desired outcomes.

In this concluding summary, the speaker reflects on the topics related to the differential amplifier that were discussed, particularly focusing on numerical calculations like operating point determination, and different types of gain calculations. Acknowledge that though the differential gain observed was modest (around 8), the implementation of an active tail resistor improved the circuit's overall effectiveness by substantially suppressing common mode signals while maintaining clear differential outputs.