66.2.6. Numerical Examples and Design Guidelines

The gain starting from the input which is getting converted into current I and then through the multiplication of β then we obtain. So, this I is getting converted into I × β and then by this load line the signal part is getting converted back into this voltage. So, I should say that we do have a voltage here, voltage it is getting converted into current and then this current is coming to this y-axis and then this load line characteristic is converting back this current into voltage. So we can see that we do have two reflectors, one is I versus V characteristic reflector multiplied by β and then we do have the other reflector.

To understand how the voltage gain of an amplifier is affected by the conversion of voltage to current and back to voltage, we look at the process step-by-step. A signal is initially input as a voltage, which influences the base current (represented as I_b). This base current is then amplified by a factor of β (the current gain of the transistor) to produce the collector current (I_c). This collector current flows through the load resistor, producing an output voltage. The effectiveness of this conversion is determined by the configuration of the amplifier and the characteristics of the load. Hence, there are crucial factors, such as the slope of the I-V characteristics of the transistor, affecting overall gain.

Think of a water pump. The water pressure at the input (voltage) activates the pump (transistor) to push more water (current) through pipes (load), and the output flow (output voltage) depends on how efficiently the pump converts input pressure to flow. The ratio of output water pressure to input pressure chances based on the pump's design, similar to how an amplifier transforms input voltage to output voltage.

Now, if you see that the gain starting from the input which is getting converted into current I and then through multiplication of β then we obtain. The gain of the amplifier it is primarily getting restricted by the voltage drop across this resistance divided by V_T.

The gain of an amplifier has practical limitations due to several factors including power supply limitations and physical component behaviors. This section emphasizes that the maximum gain is restricted by the thermal voltage (V_T), which is the voltage equivalent of thermal energy. In this context, if the circuit configuration leads to excessive voltage drops across resistors, the effective gain diminishes. Therefore, smart circuit design is necessary to navigate these limitations.

Consider trying to fill a water balloon (output voltage) while constantly leaking some water (voltage drop across the resistance). Even if you pump water in (input voltage), if there's too much leakage (excessive resistance), the final amount of water in the balloon won't be as much as you hoped. This leakage is similar to how voltage drops limit the overall efficiency and gain of an amplifier.

So, this is what we see that limitation of the voltage gain, in this circuit because the g_m is good in this circuit the gain value it is very decent. But in case if you want to further enhance then we may look for some alternative. Now what may be the alternative? Let us try to intuitively understand that what may be the scope of improvement of this gain.

The section discusses strategies to improve amplifier gain by switching from passive load components (which have inherent limitations) to active load configurations (like transistors). Active loads inherently yield better voltage gain as they can boost the output without increasing supply voltage. The essence is to replace inefficient resistor loads with transistors, which allows for greater control over the circuit's characteristics and thus enhances performance.

Imagine using a stronger motor and a pulley system to lift weights instead of simply lifting the weight by hand (passive load). The motor (active load) offers greater mechanical advantage. This means you can lift heavier weights (achieve higher gain) using the same power input (supply voltage). The motor system adjusts and responds dynamically to the load, much like active load circuits adjust voltages to optimize signal strength.

If we consider the common source amplifier, on the other hand, it is philosophically it is same, only thing is that the I-V characteristic instead of I versus V, now we have to I versus V_ds.

The common source amplifier and common emitter amplifier have a similar principle but differ in structure and the manner in which they handle input and output signals. The common source amplifier operates similarly yet from the perspective of MOSFETs, showing how each configuration translates input signals to output while maintaining or changing the I-V characteristic profiles. Understanding these variations helps in choosing the right amplifier topology for specific applications.

Consider two types of engines: one is a petrol engine (common emitter) and the other a diesel engine (common source). Both engines serve the same purpose of driving a vehicle (amplifying a signal) but have different operational characteristics, performance metrics, and efficiencies depending on the fuel and design, which can be analogous to different amplifier types handling signals differently.

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