Amplifier is one of the most commonly used design block that we can encounter in every system. What are you considering when you need to make a decision? What are the priorities in your amplifier design? These and further questions will be answered in this article and amplifier design parameters will be explained in detail.
First of all let’s start with amplifier topologies that you can select. Each of these amplifier topologies will be explained in detail in further articles.
- Single Stage Amplifiers (Common Source, Source Follower, Common Gate)
- 5T Differential Amplifier (Single ended or differential output)
- Telescopic Cascode Amplifier (Single ended or differential output)
- Folded Cascode Amplifier (Single ended or differential output)
- Class-AB Amplifiers
As listed in Amplifier Topologies chapter, there are multiple amplifier options. But what should we consider while we are making decision, there should be some design considerations that helps us to make decision. These design considerations will be exlained in detail.
DC Gain is defined as the small signal gain of the amplifier at low frequencies. If there is not any feedback mechanism, output of the amplifier will be determined by following formula;
Vout=Av x (Vinp-Vinm)
DC Voltage gain of the amplifiers depends on temperature and process since there is a gm and r0 terms in gain formulas. Therefore in most of the amplifier designs, feedback mechanism is implemented. And most of these feedback is achieved by connecting output and Minus input of amplifier that is called as Unity Gain or Buffer Amplifier.
Amplifier DC Gain is an important criteria due to the closed-loop gain of the amplifier has the following formula. If A is not too large compared to feedback gain, there will be unexpected non-linearities.
where A is open-loop gain of the amplifier, β is feedback gain, and Av is gain of the closed-loop system.
Frequency response is an important criteria that determines the speed and stability of the system. To be able to analyze the frewuency response of the system, STABILITY analysis in Cadence Spectre can be utilized. The feedback loop should be broken and IPROBE instance should be placed in broken loop. Then frequency response should be analyzed.
Phase Margin of the amplifier should be larger than 45 to prevent oscillation. To be operate in safe margin, you may try to have Phase Margin larger than 60.
Power is an important concern for system that uses batteries compared to systems with plug. Because lower power will result in longer usage without charging.
Power consumption of the system can be determined by considering slew rate specification and noise will have an impact on determining power consumption of the system.
In most of the high speed applications, output slew rate of the amplifier is critical and you do not want to have amplifier that is bottleneck of the whole system. Slew Rate is defined as the rate of rising/falling at the output. It is defined as V/sec or V/usec. To characterize Slew Rate, a large signal square pulse with very low rising-falling time is applied to PLUS input of the amplifier. Then time between %10 -%90 transition is analyzed.
Rising and falling settling time are defined as the settling of amplifier for a predefined settling criteria (such as %1 or %0.1 settling time). Settling time has similarities with Slew Rate but it will NOT be correct to say Settling is just determined by Slew Rate. Because Slew Rate is dominant for large signal setlling whereas for settling calculations, RC time constant has an effect as well.
Input and Output Swing Range
Depending on the application input and output driving capability will be critical. This will be important when making amplifier topology selection. For instance, if full swing is desired at the output, selecting Telescopic Cascode Amplifier will not be a good selection.
Input and output swing ranges are determined through the VTH and VOV drop of the transistors at the output or input stages. There are some options to increase input swing range such as adding Complementary Input Stage.
Most important consideration for the designs that requires high accuracy is noise. Noise has an inportant effect on Signal-to-Noise (SNR) of the system. In your selection if you are aiming to have more than 14 bit resolution, you need to enhance your amplifier noise.
In amplifiers, Thermal Noise and 1/f Noise are most dominant noise sources. To enhance noise performance of your system, first of all you need to find out which transistor has largest contribution and which noise source is dominant. If thermal noise dominates, w/L ratio of the amplifier should be increased. On the other hand is 1/f noise of a particular transistor dominates the noise, you should increase w*L while keeping w/L constant.
Output Resistance and Load
Output resistance is important parameter if you drive resistive load. Because driving load resistor that ahs lower resistance than amplifier output resistance will have disastrous impact on DC Gain.
Gain of an amplifier can be defined as Gm*Rout. Therefore lower resistive load will decrease overall gain. Therefore output resistance and load resistance should be calculated carefully.
In addition to resistive load, capacitive load can be problematic as well. For instance, for 2-Stage amplifier design, dominant pole is related to output of first stage and output node of second stage generates non-dominant pole. If output load has larger capacitance, there will be problem with frequency response.