Sreath 1: Circuits Biased

Chapter 1: Biasing Circuits

• Biasing circuits are used to bias discrete bipolar junction transistors (BJTs), which require a large number of resistors and bypass capacitors.
• Current biasing in integrated circuit (IC) designs uses constant current sources, generated at a single location within the IC, and reproduced at other locations in the amplifier stages.
• This approach is economically feasible compared to using large capacitors to provide the same level of biasing.

Diodes connected BJTs

• Diodes are used in conjunction with bipolar junction transistors (BJTs) in various configurations (NPN and PNP) to form circuits.
• Specific currents (Ie, Iel, Vce, etc.) are labelled on these diagrams.

Simple Current Mirror Circuit

• A current mirror circuit forces the output current to equal the supply current.
• This circuit has two matched transistors (Q1 and Q2).
• The bases and emitters of the transistors are connected together, and the collectors are shorted.

Current Mirror Circuit Specifications

• The constant current source feeding the current mirror circuit is called Iref.
• The output current (Iout) is the same as the input current (Iref).
• The current mirror circuit is effective at maintaining a constant output current, even when there are changes in the input or supply voltages.

Current Steering Circuit

• In this circuit, a reference current (Iref) is used to steer or control the flow of current to other parts of the circuit.
• Several transistors (Q1, Q2, Q3, etc.) are involved, with their connections being strategically placed to allow for controlled current distribution.
• Resistors (like Rref) act to determine the reference current's level.

Current Mirror – Improved Circuit

• Improved current mirror circuits address performance concerns, like dependence on beta (β), by minimizing the effect of variations in transistor parameters.
• Output resistance is made significantly higher, offering better performance under fluctuating conditions.
• The goal is for the error in current gain to be less evident.

Ideal Differential Amplifiers

• These amplifiers have two inputs and two outputs.
• The inputs are labelled as V1 and V2, while the outputs are labelled as V01 and V02.
• The differential input voltage (Vid) and the differential output voltage (Vod) are used to quantify differences between the two inputs and outputs.
• The voltage gain (Av) of the differential amplifier is dependent on these quantities: Vid, Vod and their associated resistors, RC, and Rin.

Differential Amplifier Analysis

• The overall output voltage of the circuit is proportional to the difference in the two input voltages (V1 and V2).
• The relationships between the input and output voltages are determined by circuit configurations and component values.
• DC considerations determine the operating point, whereas AC considerations involve signal levels.

Tha circuits biased air an cleachdadh gus BJTs a bhrosnachadh. Bidh diodes a’ cleachdadh ann an conaltradh le BJTs ann an diofar dhòighean. Tha an cuairteachadh cudromach seo a’ leigeil le cruthachadh chùisean co-chosmhail.