Anritsu HFE0303_Tutorial 电路图.pdf

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1、42High Frequency Electronics High Frequency Design TRANSISTOR BIASING Transistor Biasing Issues for Linear Amplification of Complex Signals By Gary A. Breed Editorial Director T ransistors are the workhorse of RF and microwave sig- nal amplification. They are everywhere, as dis- crete devices and in

2、corpo- rated into integrated cir- cuits.Every transistor must be provided with an operating voltage, a signal input and bias. This basic tutorial explores the last of thesebias. Bias techniques have undergone some changes in recent years to accommodate the complexity of modern communications.We will

3、 review some fundamentals, then offer an overview of issues that effect the way transis- tor biasing is managed to maintain linearity in todays wireless systems. Bias Fundamentals The purpose of bias is to establish opera- tion on the desired portion of a transistors input/output transfer function c

4、urve (Figure 1). Note that bias is almost always referenced to voltage, even though BJTs are current-con- trolled devices. In these devices, the bias volt- age establishes the base-emitter current that is the input to the device. FETs are voltage- controlled, so bias is a direct Looking at Figure 1,

5、 and remembering that the input signal is summed with the bias voltage, lets look at the operating regions: Cutoff region “C”Well start with this region, since it has the least impact on linear amplifiers. When the chosen bias sets opera- tion in the cutoff region, a large contribution is required f

6、rom the input signal to create any output at all. When no signal is present, there will be no collector or drain current (other than leakage). This situation is acceptable, even preferred in two circumstances: Class C power amplifiers and digital/switching appli- cations, including switch-mode power

7、 ampli- fiers.This on/off step function is, by definition, nonlinear.Thus, transistors biased at or below cutoff are not suitable for linear applications such as small signal amplifiers or linear power amplifiers, and we require no further discussion for this article. Saturation region “D”Each trans

8、istor has a maximum collector or drain current it can deliver. When the bias+input signal is high enough to reach this output level, the amplifier ceases to be linear. When dealing with the saturation region, we are mainly con- cerned with the maximum power available from the device. In modern wirel

9、ess systems, Here is tutorial review of biasing issues for transistors, with notes on the methods that are used to control bias in modern wireless linear power amplifiers Input Output CBA D Figure 1 A highly simplified transistor trans- fer function curve. Input is base current for a bipolar junctio

10、n transistor (BJT), gate voltage for a field-effect transistor (FET); Output is collector current for a BJT, drain current for a FET. “A” is the conduction region, “B” is the transition region, “C” is the cutoff region and “D” is the saturation region. From March 2003 High Frequency Electronics Copy

11、right 2003, Summit Technical Media, LLC RadioFans.CN 收音机爱 好者资料库 44High Frequency Electronics High Frequency Design TRANSISTOR BIASING the maximum average power is of lit- tle concern, but peak power is impor- tant. Some complex signal waveforms have average power as little as seven percent of their

12、peak power. Rather that deal with peak power as a biasing issue, most wireless amplifier designers select (or design) a transistor type that will support the required peak power. For applica- tions where it is required, there are techniques that offer one to three dB of compensation for the compress

13、ion of the signal at peak power Conduction region “A”When the bias establishes the operating point of a transistor within the conduction region, the output is a linear function of the input. Class A amplifiers are biased to remain within the conduc- tion region during the plus or minus voltage swing

14、 of the input signal.The range of the input swing is deter- mined by how far the bias point is up the curve. The cost of accommodating higher input (and output) voltage swing is higher no-signal output cur- rent. In short, class A amplifiers con- sume a lot of current, even when no input signal is p

15、resent. The main issue with the conduc- tion region is power consumption, or efficiency. Yet, this type of operation is normally chosen to obtain linear operation. Since the most advanced modulation methods used today require linear amplification, this is a major area of interest. In todays wireless

16、 systems, the main issue with power amplification is how to main- tain full class A performance while mitigating its low efficiency. Transition region “B”The tran- sition region is key to any discussion of biasing for linear operation. Semiconductor junctions never oper- ate ideally, going from non-

17、conduction to conduction completely and exactly at the threshold current or voltage. So, there is a region in the transfer curve where the output gradually increases with increasing bias. In audio and many RF applica- tions, class B amplifiers have been common for many years. These typi- cally use t

18、wo devices, operating in push-pull (180 degree phasing). Each device only needs to conduct during half of the cycle of the input signal. Each half-cycle will be amplified lin- early in its associated device, with only a small quiescent current during the “off” portion of the cycle. Although class B

19、amplifiers are much more efficient than class A, other performance issues limit lin- earity, such as phase accuracy in the 180 degree dividing and combining circuits, obtaining identical perfor- mance of the two devices, and the effects of not-quite-complete cutoff during the unused half-cycle. If w

20、e can accept these imperfect behaviors, class B push-pull amplifiers are a useful way to get linear amplification with good efficiency. Next, if we change the bias of a class A amplifier so that the swing of the input signal sometimes reaches into the transition region (class AB), we lower the quies

21、cent current, improving efficiency, but like class B push-pull, it is at the expense of some nonlinearity. Some established appli- cations can tolerate a degree of imperfection in “linear” amplifica- tion. For example, single-sideband (SSB) voice transmission can tolerate some distortion and NTSC te

22、levision signals can use relatively simple compensating circuits to minimize observable effects. Todays Linearity Issues Modern digital wireless signals are sufficiently complex that they require linear amplification with lit- tle compromise. In response to these requirements, engineers have devel-

23、oped a number of techniques to obtain an optimal balance between linearity and efficiency. The major techniques are outlined below: PredistortionThe concept of pre- distortion is to modify the input sig- nal in manner that is opposite of the effects of the amplifier nonlinearity. The amplifier trans

24、fer function can be measured and a circuit devised with the appropriate compensation. Some types of predistortion also vary the bias in proportion to the input signal. Although predistortion can be highly effective, it has the limitation that amplifier nonlinearities are not constant. They vary with

25、 tempera- ture, component aging and other fac- tors. Some of these can be addressed separately to help make predistortion more effective. Temperature compensationTo make any linearization technique more reliable, amplifiers must have temperature compensation. The task is getting harder with high pea

26、k-to- average ratio of digital signals, since the temperature of the transistor die changes much faster than the tem- perature of the heat sink! Some tran- sistors are being manufactured with an integral diode or resistive thermal sensor that is used with compensa- tion circuitry that adjusts the bi

27、as to maintain the same operating point under varying conditions. Degradation compensationAll transistors change over their operat- ing lifetime, but the popular LDMOS devices have a significant rate of change over time. Early devices even had to be replaced as they fell out of spec. Current product

28、ion LDMOS transistors have improved this per- formance problem, but most amplifi- er manufacturers have implemented some type of bias adjustment to com- pensate for the remaining perfor- mance degradation over time. There are other interesting tech- niques used for compensation of non- linearities,

29、such as feed-forward lin- earization and digital predistortion, but as they are not directly related to bias, are not discussed here. This overview has identified some of the power amplifier linearity issues faced by todays engineers. In future issues, we will provide timely technical articles on this subject that show how specific problems are being dealt with. RadioFans.CN 收音机爱 好者资料库