If the base resistor of Fig. 118 is increased to 910 k Ω, find the new Q -point and resulting va

If the base resistor of Fig. 118 is increased to 910 k Ω,...

Key Concepts

DC Operating Point (Q-Point)

The Q-point, or quiescent point, is the steady-state operating condition of a transistor in a circuit where all AC signals are absent. It is defined by the DC collector current and collector-emitter voltage, and is critical to ensure that the transistor operates in the desired region (typically the active region) for proper amplification or switching. Establishing the Q-point involves analyzing the biasing network and transistor parameters to set the correct currents and voltages under no-signal conditions.

Base Biasing Resistor

The base resistor is a key component in a transistor biasing network, as it sets the base current by limiting the current flowing into the base junction of the transistor. Changing the value of this resistor directly affects the base current according to Ohm's Law, which in turn influences the collector current (through the transistor’s current gain) and consequently the collector-emitter voltage. Adjusting the base resistor value is a common method to establish or fine-tune the transistor’s Q-point.

BJT Current Relationships

In a bipolar junction transistor (BJT) circuit, the relationship between the base current (I_B) and the collector current (I_C) is typically defined by the transistor’s current gain (?), where I_C ? ?I_B in the active region. This proportional relationship is essential for determining the DC bias point, as any change in the base current, resulting from modifications in the biasing resistor, exponentially influences the collector current and helps set the transistor’s operating conditions.

Kirchhoff's Voltage Law (KVL) in Biasing Analysis

Kirchhoff's Voltage Law is a fundamental principle used in DC circuit analysis, stating that the sum of the voltage drops around any closed loop in a circuit must equal the total supplied voltage. In the context of transistor bias circuits, KVL is applied to the base loop and the collector loop to compute the voltage drops across resistors and the junctions, allowing for the determination of the base current and the collector-emitter voltage. This systematic application of KVL is crucial in calculating the new Q-point when circuit components, such as the base resistor, are modified.

Transcript

00:01 In this problem we are going to calculate the current passing through the register r3, which we call i 3.

00:08 Let we have r1 equals to 3 .0, r2 equals to 4 .0, r3 equals to 12m, r4 equals to 6 .0, and r5 equals to 2 .0.

00:26 As we know that 2r4s are in parallel combination, so we can write, 1 by re1 equals to 1 by r4 plus 1 by r4.

00:36 Now writing the value for the r4 we get 1 by re1 equals to 1 divided by 6 .0 plus 1 divided by 6 .0.

00:46 So this will give us re1 equals to 3 .0.

00:50 Let's call it equation 1.

00:51 Now for series combination we can write re2 equals to re1 plus r2.

00:58 Let's write the values for this re1 and r2.

01:02 Re2 equals to 3 .0 plus 3 .0.

01:05 This is equals to 6 .0.

01:09 Let's call it equation 2.

01:12 As r2 and r3 are in parallel combination so we can write 1 by re3 equals to 1 divided by r2 plus 1 by r3.

01:20 Let's write the values for this r2 and r3.

01:23 1 by r3 equals to 1 divided by 4 .0 plus 1 divided by 12 om.

01:29 So this will give us re3 equals to 3 .0.

01:32 Let's call it equation 3.

01:35 Now for series combination we can write re4 equals to re3 plus r5.

01:41 Now setting the values of this re3 and r5 into this equation we get re4 equals to 3 .0 plus 2 .0...

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